I often hear an unlawful act, so what is an illegal act?
Dictionary.com defines the words as:
forbidden by law or statute.
contrary to or forbidden by official rules, regulations, etc.
- not lawful; contrary to law; illegal
so it appears they might be synonyms. Is there some nuance that I'm missing?
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.
The New Oxford American Dictionary 3rd 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 it means a person present in a country without official authorization, and which is never replaced by unlawful.
Looking at the Oxford English Dictionary, I found 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.
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...
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.
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.
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.
U.S. synonym guides focus on legal/illegal and lawful/unlawful in their statutory sense as descriptions of what is permissible or impermissible according to a code of laws. The result, unfortunately, is not very satisfactory coverage of the terms as they are more broadly used in the real world.
For example, S.I. Hayakawa, Modern Guide to Synonyms and Related Words (1968) has this entry for lawful and legal (which it considers as part of a bundle of related words that also includes constitutional, legitimate, and licit):
Lawful implies conformity with laws, statutes, canons, precepts, principles, rules, etc. intended to regulate the conduct of those coming within their particular field of action. Thus one speaks of lawful debts, a lawful claim, a lawful marriage, of conducting a lawful business or making a lawful decision. Legal has nearly the same meaning, but is restricted chiefly to statute laws as they apply at certain times and places {Divorce is lawful but subject to various legal requirements before taking final effect; The legal speed limit within the town is 15 miles per hour.} ... Antonyms: illegal, ... unlawful.
And Merriam-Webster's Dictionary of Synonyms (1984) has this for unlawful and illegal:
unlawful, illegal, illegitimate, illicit are comparable when they mean contrary to, prohibited by, or not in accordance with law or the law. Otherwise than this negation in character, the words in general carry the same differences in implications and connotations as the affirmative adjectives discriminated at LAWFUL.
and this for lawful and legal:
lawful, legal, legitimate, licit mean permitted, sanctioned, or recognized by law or the law. Lawful differs from the others in implying a reference to various sorts of law (as divine law, natural law, or the law of the land, or as civil law, common law, or canon law). Consequently, the term often comes close in meaning to allowable or permissible {all things are lawful unto me,but all things are not expedient—1 Cor[inthians] 6:12} {tell me, which knave is lawful game, which not? Must great offenders, once escaped the Crown, like royal harts, be never more run down?—Pope} or sometimes to rightful or proper {the lawful heir} {a lawful prize} {the lawful sovereign} {that man was not Hannah's lawful husband—Ingamells} {William desired to reign not as a conqueror but as a lawful king—J. R. Green} Legal implies a reference to the law as it appears on the statute books or is administered in the courts; thus, the lawful heir is also the legal heir; the lawful owner of a piece of property is one whose legal right to to it is certain; a moneylender is entitled only to legal interest on his loans. Legal is used more often in the sense of sanctioned by law or in conformity with the law, or not contrary to the law, than in the sense of allowable by the terms of the law {a legal marriage} {the legal period for the payment of a debt} {the capture of the neutral ship carrying contraband was held to be legal} {she became the virtual head of our family, supplanting ... my Uncle Tiberius (the legal head)—Graves {the Vichy regime he considers an illegitimate, although, at first at least, it was outwardly legal—Guerard}
All of this may be accurate as applied to criminal and civil codes of law, but, as tankadillo points out in a rather underappreciated answer posted here ten years ago, use of illegal to refer to conduct outside the world of actual laws is extremely common, at least in the United States. In particular, multiple sports use the term illegal, not unlawful, to refer to various infractions of the rules. Thus, in U.S. football, penalties may be assessed for "illegal use of hands," an "illegal forward pass," an "illegal shift," an "illegal formation," or (more broadly) "illegal procedure"; in U.S. basketball (as in U.S. football), a referee may call a foul for "illegal defense" or an "illegal timeout"; and in U.S. baseball, a pitcher or batter may be penalized for applying an "illegal substance" to the ball or bat. In none of these cases would the word "unlawful" ever be used in place of "illegal" because in none of these cases is the infraction against any law; each simply involves violating the rules of the game.
Similarly (as tankadillo notes) a computer operating system may inform the user that "This program has performed an illegal operation and will be shut down." Essentially the operating system didn't understand or didn't approve of the program's behavior, so it closed the program. Likewise, none other than the Oxford Dictionary of Computing, sixth edition (2008) includes entries for "illegal character" and "illegal instruction":
illegal character Any character not in the character set of a given machine or not allowed by a given programming language or protocol.
illegal instruction An instruction that has an invalid operating code. It is sometimes deliberately inserted in an instruction stream when debugging in order to have a program halt, or interrupt, at a particular point.
Again, "unlawful character" and "unlawful instruction" are not in the lexicon because they aren't in real-world use.
In all of these areas, U.S. usage is exactly the opposite of what one might expect from the OED usage note cited in apaderno's answer: whereas the OED indicates that illegal narrowly implies "against formal statutory law" and lawful more broadly implies "contravening applicable rules," lay/informal U.S. usage treats unlawful as more strictly concerned with actual law (of some kind) and illegal as more broadly concerned with the normal and approved rules of play or operation.
