P

Pacta sunt servanda
Translation: "Agreements must be honoured."
Parturiunt montes, nascetur ridiculus mus
Translation: "The mountains are in labour, and a ridiculous mouse shall be born" — i.e. "much ado about nothing"; from Horace.
Pax melior est quam iustissimum bellum.
Translation: "Peace is better than the most just war."
Pecunia non olet.
Translation: "Money does not smell." (Remark by Roman emperor Vespasian on the plan to tax public urinals.)
Peior est bello timor ipse belli.
Translation: "Worse than war is the very fear of war."
Per aspera ad astra
Translation: "Through hardships to the stars" (motto of NASA) from Seneca.
Per fas et nefas
Translation: "With right and wrong" by any means necessary
Per scientiam ad salutem aegroti.
Translation: "To heal the sick through knowledge."
Perditio tua ex te, Israel
Translation: "Destruction is thy own, Israel" (Bible, Hosea IX:13)
Periculum in mora
Translation: "[There's] danger in delay" (Livy)
Perspecite potestatem caesi.
Translation: "Behold the power of cheese."
Piscem natare doces
Translation: "[You] teach a fish to swim."
Piscis primum a capite foetet
Translation: "Fish stinks from the head first"
Plenus venter non studet libenter.
Translation: "A full belly doesn't like studying."
Plures crapula quam gladius perdidit.
Translation: "Drunkenness takes more lives than the sword."
Post cenam non stare sed mille passus meare.
Translation: "Do not rest after dinner, but walk a mile."
Post hoc non est propter hoc.
Translation: "'After this' is not 'because of this'."
Post Tenebras Lux
Translation: "After the darkness the light" (motto of the canton Geneva, Switzerland)
Potius sero quam numquam
Translation: "Better late then never" (Livy)
Praemonitus, praemunitus
Translation: "Forewarned (is) forearmed"
Praesente medico nihil nocet.
Translation: "In the presence of a doctor nothing can harm."
Praevenire melius est quam praeveniri.
Translation: "It is better to precede than to be preceded."
Primum ego, tum ego, deinde ego.
Translation: "First I, then I, thereafter I." (The author of this confident statement, a Roman emperor, will be added soon!)
Primum non nocere
Translation: "First, do no harm" (often falsely attributed to the Hippocratic Oath).
Principiis obsta
Translation: "Resist the beginnings" (i.e. undesirable trends should be nipped in the bud).
Pro aris et focis
Translation: "For altar and hearth" i.e. for our homes (Cicero)
Pro Deo et patria
Translation: "For God and Country" (Unknown)
Proximus sum egomet mihi
Translation: "I am closest to myself" (Terence)
Pulvis et umbra sumus
Translation: "We are dust and shadow" (Horace, Carmina, Book IV, 7, 16).

Q

Quæ communiter possidentur communiter negliguntur
Translation: "(Things) which are possessed in community are neglected in community."
Qualis rex, talis grex
Translation: "Like king, like people"
Quem di diligunt, adulescens moritur
Translation: "Whom the gods love dies young" (Plautus, Bacchides, IV, 7, 18). In the comic play, a sarcastic servant says this to his aging master. The rest of the sentence reads: dum valet, sentit, sapit, "while he is full of health, perception and judgement."
Quem dii odere, paedagogum fecere (also Quem dii oderunt, paedagogum fecerunt)
Translation: "Whom the gods hated, they made them pedagogues"
Qui dormit non peccat.
Translation: "He who sleeps does not sin"
Qui habet aures audiendi audiat
Translation: "Those who have ears to hear, hear!" (Vulgate, Matthew 11:15)
Qui rogat, non errat.
Translation: "(One) who asks, doesn't err."
Qui scribit, bis legit.
Translation: "Who writes, reads twice."
Qui tacet, consentire videtur.
Translation: "Who is silent seems to agree."
Qui transtulit sustinet.
Translation: "He who is transplanted is still sustained." (motto of Connecticut referring to the transplantation of settlers from England to the New World.)
Qui vult dare parva non debet magna rogare.
Translation: "He who wishes to give little shouldn't ask for much."
Quia suam uxorem etiam suspiciore vacare vellet.
Translation: "Caesar's wife may not be suspected" (Plutarch, Caesar 10) The rhetorian Clodius was having an affair with Caesar's second wife, Pompeia. At a party attended by Pompeia Clodius arrived in disguise but was caught. In the following trial, Caesar claimed that nothing wrong had happened, but he still had to divorce her.
Quid Saulus inter prophetas?
Translation: "What is Saul doing among the prophets?" (a fifth wheel)
Quidquid agis, prudenter agas, et respice finem!
Translation: "Whatever you do, may you do it prudently, and toe the line!"
Quidquid discis, tibi discis
Translation: "Whatever you learn, you learn it for yourself."
Quidquid id est timeo puellas et oscula dantes.
Translation: "Whatever it is, I fear the girls, even those giving kisses." (a variant on Timeo Danaos et dona ferentes).
Quidquid latine dictum sit, altum videtur.
Translation: "Anything said in Latin sounds profound."
Quieta non movere
Translation: "Don't move settled things" (i.e. "Don't rock the boat").
Quis custodiet ipsos custodes?
Translation: "Who will watch the watchmen themselves?" (Juvenal).
Quod erat demonstrandum.
Translation: QED "Which was to be demonstrated."
Quod licet Iovi, non licet bovi.
Translation: "All that is allowed to Jupiter is not necessarily allowed to an ox."
Quod medicina aliis, aliis est acre venenum.
Translation: "What is medicine to some, is bitter poison to others."
Quod nocet, saepe docet
Translation: "That which harms, often teaches"
Quod non est in actis, non est in mundo
Translation: "What is not in the documents does not exist"
Quos amor verus tenuit, tenebit.
Translation: "Those whom true love has held, it will go on holding." - Seneca
Quot capita, tot sententiae.
Translation: "As many opinions as people."
Quot linguas calles, tot homines vales.
Translation: "You are worth as many people as there are languages that you speak."

R

Radix malorum est cupiditas
Translation: "Greed is the root of all evil." (theme of the Pardoner's Tale from the Canterbury Tales)
Recta linea brevissima, recta via tutissima
Translation: "Straight line is the shortest, straight road is the most safe."
Reddite ergo quae sunt Caesaris, Caesari
Translation: "Then give Caesar what's Caesar's" (w:Vulgate:, Matthew 22:21 as well as Luke 20:25)
Repetita iuvant.
Translation: "Repetition is useful", or "Repeating things helps".
Repetitio est mater studiorum.
Translation: "Repetition is the mother of study."
Rete non tenditur milvio
[size=1]Translation: "The net is not extended to the kite" (i.e. things (of the air) fall where they may).
Ridendo dicere verum
Translation: "To tell the truth while laughing (i.e., joking)"
Risus abundat in ore stultorum
Translation: "Laughs are plentiful in the mouth of the foolish."
Romani ite domum
Translation: "Romans Go Home!" Monty Python's Life of Brian
Rustica progenies semper villana fuit.
Translation: "A rustic (as in, provincial, peasant-like) ancestry will always remain rustic."

S

Saepe morborum gravium exitus incerti sunt.
Translation: "The effects of serious illnesses are often unknown."
Salus aegroti suprema lex.
Translation: "The well-being of the patient is the most important law."
Salus populi suprema lex esto
Translation: "Let the welfare of the people be the supreme law" (motto of the U.S. state of Missouri).
Sapere aude
Translation: "Dare to be wise." (Horace)
Sapiens omnia sua secum portat
Translation: "A wise man takes everything he owns with himself" (i.e. in his head, his wealth is his wisdom)
Scio me nihil scire
Translation: "I know that I know nothing" (Socrates)
Scire aliquid laus est, pudor est nihil discere velle.
Translation: "It is commendable to know some things, it is disgraceful to refuse to learn." (Seneca)
Semper fidelis
Translation: "Always faithful", motto of the United States Marine Corps
Si decem habeas linguas, mutum esse addecet.
Translation: "Even if you had ten tongues, you should hold them all."
Si Deus pro nobis, quis contra nos?
Translation: "If God is with us, who can be against us", (Vulgate, Romans 8:31)
Si fueris Romae, Romano vivito more, si fueris alibi, vivito sicut ibi.
Translation: "If you are in Rome, live in the Roman way, if you are somewhere else, live like there."
Sine scientia ars nihil est.
Translation: "Art without knowledge is nothing." (Art and knowledge are tightly intervowen and could not exist one without the other. Source: w:Jean Vignot, w:1390?)
Si quaeris peninsulam amoenam circumspice
Translation: "If you seek a pleasant peninsula, look about you" (the motto of the U.S. state of Michigan).
Si tacuisses, philosophus mansisses.
Translation: "If you had kept your silence, you would have stayed a philosopher." Can be used as a trap for those who don't know Latin, as was demonstrated in TV sitcom Yes, Prime Minister.
Si uno adhuc proelio Romanos vincemus, funditus peribimus!
Translation: "Another victory like that, and I'm done for!" (literally, "If we defeat the Romans in a battle like this, we will completely perish.") (Plutarch, Pyrrhus 21, 14) Attributed to King Pyrrhus of Epirus after a victory with heavy casualties. See Pyrrhic victory
Si vis amaria, ama
Translation: "If you want to be loved, love" (Seneca)
Si vis pacem, para bellum.
Translation: "If you want peace, prepare for war." (Vegetius, Epitoma rei militaris) origin of the name parabellum for some ammunition and firearms, e.g. Luger parabellum
Si vis pacem, para iustitiam.
Translation: "If you want peace, prepare justice."
Sic gorgiamus allos subjectatos nunc
Translation: "We gladly feast on those who would subdue us" (motto of The Addams Family).
Sic semper tyrannis
Translation: "Thus always to tyrants" (motto of the U.S. state of Virginia; attributed to assassin Brutus, perhaps John Wilkes Booth also).
Sic transit gloria mundi.
Translation: "Thus passes the glory of the world." In Bible; repeated during the coronation of the Pope.
Silent enim leges inter arma
Translation: "Laws are silent in times of war"
Similia similibus curantur.
Translation: "Like cures like." - Samuel Hahnemann
Sine labore non erit panis in ore.
Translation: "Without work there won't be any bread in your mouth."
Sit tibi terra levitas (S.T.T.L.)
Translation: "May the earth rest lightly on you" — a benediction for the dead, often inscribed on tombstones or other gravestones.
Sol lucet omnibus
Translation: "The sun shines for everyone" (Gaius Petronius Arbiter, Satyricon 100)
Soli Deo gloria
Translation: "Glory to God alone"
Stat sua cuique dies
Translation: "The date is set for each and everyone" (Virgil)
Summum ius summa inuria.
Translation: "More law, less justice." (Cicero, De officiis I, 10, 33)
Sunt facta verbis difficiliora
Translation: "Works are harder than words." i.e. "Easier said than done."
Sunt pueri pueri pueri puerilia tractant
Translation: "Kids are kids and kids will act like kids."
Sutor, ne ultra crepidam!
Translation: "Cobbler, no further than the sandal!" I.e. don't offer your opinion on things that are outside your competence. It is said that the Greek painter Apelles once asked the advice of a cobbler on how to render the sandals of a soldier he was painting. When the cobbler started offering advice on other parts of the painting, Apelles rebuked him with this phrase (but in Greek).
Suum cuique
Translation: "To each what he deserves"

T

Tarde venientibus ossa.
Translation: "For those who come late, only the bones."
Temet nosce
Translation: "Know yourself" (from the Greek gnothi seauton, on the Temple of Apollo at Delphi). See also: Nosce te ipsum!
Tempora mutantur et nos mutamur in illis.
Translation: "The times are changed, and we are changed in them." -- w:Cicero
Tempori parce!
Translation: "Save time!"
Tempus fugit
Translation: "Time flees" (i.e., "time flies"). Originally as Sed fugit interea, fugit irreparabile tempus^**
Translation: "Meanwhile the irreplaceable time flees" (Virgil)
Teneo te, Africa!
Translation: "I have you, Africa!" Svetonius attributes this to Caesar, when the emperor was on the African coast.
Timeo Danaos et dona ferentes
Translation: "I fear the Danaens [the Ancient Greeks] even if they bring presents" (Virgil, Æneis, 2, 49) Uttered by Laocoön as he warns his fellow Trojans against accepting the Trojan Horse.
Tres faciunt collegium.
Translation: "Three makes a company."
Tu quoque Brute filii mihi?
Translation: "Even you Brutus, my son?" attributed to Julius Caesar at the 15th March after being fatally wounded.
Tunc tua res agitur, paries cum proximus ardet
Translation: "It also concerns you when the nearest wall is burning"

U

Ubi bene, ibi patria
Translation: "Where one feels good, there is one's country."
Ubi concordia, ibi victoria.
Translation: "Where there is harmony, there is victory."
Ubi dubium, ibi libertas.
Translation: "Where there is doubt, there is freedom."
Ubi fumus, ibi ignis.
Translation: "Where there's smoke, there's fire."
Ubi maior, minor cessat.
Translation: "When the bigger (greater, older) speaks, the less (younger) quits (speaking)"
Ubi mel ibi apes
Translation: "Where there's honey, there are bees."
Ubi tu Gaius, ibi ego Gaia.
Translation: "Where you are, Gaius, there I, Gaia, will be. (This is said to have been a nuptial formula, but it is only known from Greek sources.)
Ultra posse nemo obligatur
Translation: "Nobody is bound beyond ability"
Ulula cum lupis, cum quibus esse cupis.
Translation: "Who keeps company with wolves, will learn to howl."
Una hirundo non facit ver
Translation: "One swallow doesn't make spring"
Una salus victis, nullam sperare salutem
Translation: "The only [hope of ]safety for the defeated is to relinquish all hope of safety." (Virgil, Aeneid, II, 354)
Unum castigabis, centum emendabis.
Translation: "If you reprove one error, you will correct a hundred."
Usus magister est optimus.
Translation: "Practice makes perfect."
Ut ameris, amabilis esto.
Translation: "Be amiable, then you'll be loved."
Ut desint vires, tamen est laudanda voluntas
Translation: "Even if the powers are missing, the will deserves praise" (Ovid)
Ut incepit fidelis, sic permanet.
Translation: "Loyal she began, and loyal she remains" (motto of Ontario).
Ut sementem feceris, ita metes.
Translation: "You'll reap what you sow." (Cicero, "De oratore")
Ut sis nocte levis, sit cena brevis!
Translation: "That your sleeping hour be peaceful, let your dining hour be brief!" (Sis is one hour before sunset.)

V

Vae Victis
Translation: "Woe to the conquered." Attributed by Livy to the chief of the Gauls as they sacked Rome in 390 BC.
Vanitas vanitatum et omnia vanitas
Translation: "Vanity of vanities and everything is vanity." (Vulgate, Ecclesiastes 1:2)
Varitatio delectat
Translation: "Change pleases."
Varium et mutabile semper femina
Translation: "Woman is always a changeable and capricious thing."
Vasa vana plurimum sonant
Translation: "Empty pots make the most noise"
Venies sub dentem
Translation: "You will come under [my] tooth."
Ventis secundis, tene cursum.
Translation: "Go even against the flow."
Verba docent, exempla trahunt.
Translation: "Words instruct, illustrations lead."
Verba volant, scripta manent.
Translation: "Words fly, written stays."
Veritas odium paret
Translation: "Truth creates hatred" (Terence, Andria 68)
Veritas vos liberabit
Translation: "The truth will set you free"
Veritatem dies aperit.
Translation: "Time discloses the truth."
Vestigia terrent
Translation: "The traces deter" (Horace) Refers to the old fable of the wolf who refused an offer to enter the lion's den as he saw many traces leading into it, but none out.
Victrix causa diis placuit sed victa Catoni
Translation: "The victorious cause was pleasing to the Gods, but the lost cause to Cato" (Lucanus, Pharsalia 1, 128) (Dedication on the south side of the Confederate Memorial at Arlington National Cemetery)
Video meliora proboque deteriora sequor
Translation: "I see the better and acknowledge it, but I follow the worse (Ovid)
Videre videnda
Translation: "See what should be seen."
Vincere scis, Hannibal, victoria uti nescis.
Translation: "You know how to win victory, Hannibal, you do not how to use it." According to Livy a cavalry colonel told Hannibal this after the victory at Cannae in 216 BC, meaning that Hannibal should have marched on Rome directly
Vincit omnia veritas.
Translation: "Truth conquers all."
Vincit qui patitur.
Translation: "He who perseveres, conquers."
Vinum et musica laetificant cor
Translation: "Wine and music delight the heart" - Vulgate, Ecclesiasticus 40:20
Vir prudens non contra ventum mingit.
Translation: "Wise man does not urinate towards the wind."
Virtus sola nobilitat
Translation: "Virtue alone ennobles" - motto of Waverley College NSW, Australia.
Virtus, non copia vincint
Translation: "Courage, not multitude, wins"
Vis Unita Fortior.
Translation: "United strength is stronger."
Vita brevis, ars longa
Translation: see Ars longa, vita brevis
Volenti non fit iniuria
Translation: "To a willing person one cannot do injustice."
Vox audita perit littera scripta manet.
Translation: "The spoken word perishes, the written words remain."
Vox populi, vox dei.
Translation: "The voice of the people is the voice of God."
Vulpes pilum mutat, non mores!
Translation: "A fox may change its skin but never its character" - Suetonius