True law is correct reason congruent with nature, spread among all persons, constant, everlasting. It calls to duty by ordering; it deters from mischief by forbidding. Nevertheless, it does not order or forbid upright persons in vain, nor does it move the wicked by ordering or forbidding. It is not holy to circumvent this law, nor is it permitted to modify any part of it, nor can it be entirely repealed. In fact we cannot be released from this law by either the senate or the people. No Sextus Aelius should be sought as expositor or interpreter. There will not be one law at Rome, another at Athens, one now, another later, but one law both everlasting and unchangeable will encompass all nations and for all time. And one god will be the common teacher and general, so to speak, of all persons. He will be the author, empire, and provider of this law. The person who will not obey it will flee from himself, and, defying human nature, he will suffer the greatest penalties by this very fact, even if he escapes other things that are thought to be punishments.

[Est quidem vera lex recta ratio naturae congruens, diffusa in omnes, constans, sempiterna, quae vocet ad officium iubendo, vetando a fraude deterreat; quae tamen neque probos frustra iubet aut vetat nec improbos iubendo aut vetando movet. Huic legi nec obrogari fas est neque derogari ex hac aliquid licet neque tota abrogari potest, nec vero aut per senatum aut per populum solvi hac lege possumus, neque est quaerendus explanator aut interpres eius alius, nec erit alia lex Romae, alia Athenis, alia nunc, alia posthac, sed et omnes gentes et omni tempore una lex et sempiterna et immutabilis continebit, unusque erit communis quasi magister et imperator omnium deus, ille legis huius inventor, disceptator, lator; cui qui non parebit, ipse se fugiet ac naturam hominis aspernatus hoc ipso luet maximas poenas, etiamsi cetera supplicia, quae putantur, effugerit.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

De Re Publica [On the Republic, On the Commonwealth], Book 3, ch. 22 / sec. 33 (3.33) (54-51 BC) [tr. Fott (2014)]
    (Source)

Stoic definition of the law. Fragment, quoted by Lactantius, Divinae Institutiones, 6.8.6-9. (Source (Latin)). Alternate translations:

There is indeed a law, right reason, which is in accordance with nature; existing in all, unchangeable, eternal. Commanding us to do what is right, forbidding us to do what is wrong. It has dominion over good men, but possesses no influence over bad ones. No other law can be substituted for it, no part of it can be taken away, nor can it be abrogated altogether. Neither the people or the senate can absolve us from 38it. It wants no commentator or interpreter. It is not one thing at Rome, and another thing at Athens: one thing to-day, and another thing to-morrow; but it is a law eternal and immutable for all nations and for all time. God, the sole Ruler, and universal Lord, has framed and proclaimed this law. He who does not obey it, renounces himself, and is false to his own nature: he brings upon himself the direst tortures, even when he escapes human punishments.
[tr. Featherstonhaugh (1829)]

There is a true law, a right reason, conformable to nature, universal, unchangeable, eternal, whose commands urge us to duty, and whose prohibitions restrain us from evil. Whether it enjoins or forbids, the good respect its injunctions, and the wicked treat them with indifference. This law cannot be contradicted by any other law, and is not liable either to derogation or abrogation. Neither the senate nor the people can give us any dispensation for not obeying this universal law of justice. It needs no other expositor and interpreter than our own conscience. It is not one thing at Rome and another at Athens; one thing to–day and another to–morrow; but in all times and nations this universal law must for ever reign, eternal and imperishable. It is the sovereign master and emperor of all beings. God himself is its author, -- its promulgator, -- its enforcer. He who obeys it not, flies from himself, and does violence to the very nature of man. For his crime he must endure the severest penalties hereafter, even if he avoid the usual misfortunes of the present life.
[tr. Barham (1841)]

True law is right reason conformable to nature, universal, unchangeable, eternal, whose commands urge us to duty, and whose prohibitions restrain us from evil. Whether it enjoins or forbids, the good respect its injunctions, and the wicked treat them with indifference. This law cannot be contradicted by any other law, and is not liable either to derogation or abrogation. Neither the senate nor the people can give us any dispensation for not obeying this universal law of justice. It needs no other expositor and interpreter than our own conscience. It is not one thing at Rome, and another at Athens; one thing to-day, and another to-morrow; but in all times and nations this universal law must forever reign, eternal and imperishable. It is the sovereign master and emperor of all beings. God himself is its author, its promulgator, its enforcer. And he who does not obey it flies from himself, and does violence to the very nature of man. And by so doing he will endure the severest penalties even if he avoid the other evils which are usually accounted punishments.
[tr. Yonge (1853)]

True law is right reason in agreement with nature; it is of universal application, unchanging and everlasting; it summons to duty by its commands, and averts from wrongdoing by its prohibitions. And it does not lay its commands or prohibitions upon good men in vain, though neither have any effect on the wicked. It is a sin to try to alter this law, nor is it allowable to attempt to repeal any part of it, and it is impossible to abolish it entirely. We cannot be freed from its obligations by senate or people, and we need not look outside ourselves for an expounder or interpreter of it. And there will not be different laws at Rome and at Athens, or different laws now and in the future, but one eternal and unchangeable law will be valid for all nations and all times, and there will be one master and ruler, that is, God, over us all, for he is the author of this law, its promulgator, and its enforcing judge. Whoever is disobedient is fleeing from himself and denying his human nature, and by reason of this very fact he will suffer the worst penalties, even if he escapes what is commonly considered punishment.
[tr. Keyes (1928)]

There is in fact a true law -- namely, right reason -- which is in accordance with nature, applies to all men, and is unchangeable and eternal. By its commands this law summons men to the performance of their duties; by its prohibitions it restrains them from doing wrong. Its commands and prohibitions always influence good men, but are without effect upon the bad. To invalidate this law by human legislation is never morally right, nor is it permissible ever to restrict its operation, and to annul it wholly is impossible. Neither the senate nor the people can absolve us from our obligation to obey this law, and it requires no Sextus Aelius to expound and interpret it. It will not lay down one rule at Rome and another at Athens, nor will it be one rule today and another tomorrow. But there will be one law, eternal and unchangeable, binding at all times upon all peoples; and there will be, as it were, one common master and ruler of men, namely God, who is the author of this law, its interpreter, and its sponsor. The man who will not obey it will abandon his better self, and, in denying the true nature of a man, will thereby suffer the severest of penalties, though he has escaped all the other consequences which men call punishment.
[tr. Sabine/Smith (1929)]

... law in the proper sense is right reason in harmony with nature. It is spread through the whole human community, unchanging and eternal, calling people to their duty by its commands and deterring them from wrong-doing by its prohibitions. When it addresses a good man, its commands and prohibitions are never in vain; but those same commands and prohibitions have no effect on the wicked. This law cannot be countermanded, nor can it be in any way amended, nor can it be totally rescinded. We cannot be exempted from this law by any decree of the Senate or the people, nor do we need anyone else to expound or explain it. There will not be one such law in Rome and another in Athens, one now and another in the future, but all peoples at all times will be embraced by a single eternal and unchangeable law; and there will be, as it were, one lord and master of us all -- the god who is the author, proposer, and interpreter of that law. Whoever refuses to obey it will be turning his back on himiself. Because he has denied his nature as a human being he will face the gravest penalties for this alone, even if he succeeds in avoiding all the other things that are regarded as punishments ...
[tr. Rudd (1998)]

True law is right reason, consistent with nature, spread through all people. It is constant and eternal; it summons to duty by its orders, it deters crime by its prohibitions. Its orders and prohibitions to good people are never given in vain; but it does not move the wicked by these orders or prohibitions. It is wrong to pass laws obviating this law; it is not permitted to abrogate any of it; it cannot be totally repealed. We cannot be released from this law by the senate or the people, and it needs no exegete or interpreter like Sextus Aelius. There will not be one law at Rome and another at Athens, one now and another later; but all nations at all times will be bound by this one eternal and unchangeable law, and the god will be the one common master and general (so to speak) of all people. He is the author, expounder, and mover of this law; and the person who does not obey it will be in exile from h imself. Insofar as he scorns his nature as a human being, but his very fact he will pay the greatest penalty, even if he escapes all the other things that are generally regarded as punishments.
[tr. Zetzel (1999)]