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I often hear an unlawful act, so what is an illegal act?

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For all practical purposes, they are synonyms. Various sources describe possible minor differences, such as that illegal acts are criminal acts, whereas unlawful acts may be contrary to some non-criminal law, like tort law or contract law; however, if you check actual usage, I doubt whether you will find much of a pattern in that regard. The Oxford English Dictionary gives one as a synonym of the other. Unlawful obviously comes from un- and law; illegal comes from Latin in- (which means "un-") and lex ("law"). I believe unlawful is mostly just a more formal or technical synonym. Usage may very well vary in different countries, since each country has its own legal system, even though Anglo-Saxon systems are often much alike.

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I will add that "unlawful" is rarely used in ordinary speech (or writing). It tends to be a technical term of the law. – Colin Fine Feb 28 '11 at 14:29
@Colin: Right, I was considering whether I'd add something like that, then forgot. Will add it. – Cerberus Feb 28 '11 at 14:33
Unlawful and lawful are good, stout Anglo-Saxon words that tend to be of the law -- that is, they are used in statute -- while illegal and legal are fine, robust Latinate words that have historically tended to be about the law -- they're lawyer talk, full of baloney (Bologna). – bye Feb 28 '11 at 17:22
@Stan: So you are suggesting an opposition between used in statute and lawyer talk? – Cerberus Feb 28 '11 at 17:26
Traditionally, there has been a difference. Statutes (laws) are created, discussed and voted upon by "ordinary" folk -- some are lawyers by profession, no doubt, but not all by any means. The language of laws tends to be plain. The language of law, on the other hand, is complex and technical, and carries with it the baggage of nearly a millennium of history of resolving disputations and ambiguity. (Don't read anything prescriptive into what I've said; it's just historical observation. Having written a few, I've had to look at both how "people" read laws and how lawyers do.) – bye Feb 28 '11 at 20:26

If something is unlawful, it means it is against the law, but not necessarily a criminal act; it can be a civil wrong, such as trademark infringement, for which the wrongdoer may be sued, but will unlikely face criminal prosecution.

Illegal describes an act that is unlawful and also a criminal act, such as drug trafficking.

EDIT: It appears these definitions aren't so cut and dry. This article discusses their usage in greater detail...

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Ha, it seems we both posted our answers at the nearly the same time, and made similar edits equally fast. // P.S. Are you quite sure that this difference between criminal and non-criminal holds up? I have heard it mentioned but I have seen countless example that seemed to contradict it... or is my impression truly wrong? Perhaps it also depends on the country? – Cerberus Feb 28 '11 at 14:27
Good point, see my edit... – The English Chicken Feb 28 '11 at 14:34
+1 for the Grammar Girl article. Very interesting. – tankadillo Feb 28 '11 at 16:34

The NOAD (third edition) describes illegal as "contrary to or forbidden by law, especially criminal law", and describes unlawful as "not conforming to, permitted by, or recognized by law or rules." In American English, then, illegal is used in phrases like illegal alien, where is never replaced by unlawful.

Looking at the OED, I find a note about the usage of illegal and unlawful.

Illegal and unlawful have slightly different meanings, although they are often used interchangeably. Something that is illegal is against the law, whereas an unlawful act merely contravenes the rules that apply in a particular context. Thus handball in soccer is unlawful, but it is not illegal. A third word with a similar meaning is illicit: this tends to encompass things that are forbidden or disapproved of by custom or society, as in an illicit love affair.

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This seems to be directly contrary to jjackson's answer. In the world of software, I would certainly agree with jjackson rather than the OED, but I don't know about it in games. I suspect that the comment from the OED is now somewhat outdated. – Colin Fine Feb 28 '11 at 18:25
+1, These are roughly the definitions we were given when studying business law. Illegal = Explicitly against the rules (same as in programming). Unlawful = No laws explicitly permit this. There is a verdict in the UK of "unlawful killing", although it's illegal to kill, the verdict doesn't imply murder, it could be manslaughter or dangerous driving. – Lee Kowalkowski Feb 28 '11 at 22:09
@LeeKowalkowski But, if one said "illegal killing", that would also not imply murder, too, no? It could also mean manslaughter or causing death by dangerous driving, right? – Neo Jul 11 '14 at 17:09
@Neo: I don't think a UK court is going to return a verdict of "illegal killing" though. I don't understand your comment. – Lee Kowalkowski Jul 11 '14 at 17:14
@LeeKowalkowski I wasn't necessarily referring to a verdict a court might return. I was commenting on the fact you said the term "unlawful killing" doesn't imply murder. Neither does "illegal killing". That's all. – Neo Jul 11 '14 at 21:53

Very occasionally (C.S. Lewis?), one hears "unlawful" used in the sense of 'Moral Law', as opposed to 'man made' laws. Otherwise, for practical purposes synonymous, as already stated.

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One difference to note is that unlawful is generally only used in the context of state or federal laws, wereas illegal can be used to refer to any set of rules. For example, in sports people can perform illegal moves, and when a computer program crashes it will sometimes say that it performed an illegal operation.

Within the context of the law though, they are much more synonymous save for the differences that the other answers mention.

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The answer by kiamlaluno says the opposite. – Anixx Apr 24 '12 at 18:06
@anixx: The program has performed an unlawful operation? Nope. You may try to perform an illegal move in chess, unless your move is drawing a gun on your opponent. Punching below the waist line in boxing is illegal, not unlawful. Simply, 'illegal' can apply to arbitrary set of rules, while 'unlawful' applies to law. – SF. Jul 7 '14 at 12:38

This is an important definition and so far we've not really had it from the comments.

The true definition is within the context of the difference between Statute and Constitutional Law, yes despite baseless assertions to the contrary, we British and all Common Law jurisdictions have one, that’s why we have a Constitutional Monarchy. Constitutional Law includes Constitutional Instruments such as in 1215 Magna Carta and 1688 Declaration of Rights, both of which are a part of the US Constitution in addition to the US Constitution itself and Common Law. These are the only things that are Law per se. There is no such thing as Statute Law, the term is a misnomer, more properly it should be Statute Legislation, which can only be lawful if it follows the superiority of Common Law. So ‘legal’ refers to statute legislation and ‘lawful’ refers to constitutional and common law. Lawful is higher than legal.

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Illegal is a word used generally in a explicit negativeness where as unlawful is not to sound as derogatory.

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Could you add some corroboration, or even examples, to back up a naked assertion? – Andrew Leach Jul 7 '14 at 12:50

protected by tchrist Jul 7 '14 at 13:59

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