Illegal, is an adjective deriving directly, in terms of etymology, from Latin Lex=Law, 'Legis'=of Law, and indicates an act, or state and situation that is/are against Law, not in accordance with Law, and its rule in a specific domain and field of Law. Thus, we can say that a penal act is an illegal act, that a crime is illegal act, as penal act first, against Law, and is punished/punishable from Law.

The opposite term is legal, thus legitimate and illegal=illegitimate,

for ex. 'today smoking in airplane during flight is illegal, thus prohibited by authorities, as something against their principles, and rules of Statute.

In all fields of Law, incl. for ex. Civil Law, Law of Labour, etc, we say that something is 'illegal' when not accepted by Law. For ex. in Civil Law, 'act of tort' often involves illegal/non legal, acts, often involving fraud. (fraudulent acts are illegal acts..). Illegal is usually immoral, but in our today legal culture, something accepted for ex. by local societies and ethics (Custom Law) may not be moral, while being legal at the same time.

Also we have 'legal job' thus 'legitimate job' job acceptable by State as legal, thus valid one, in terms of basic rules of Constitution, (Natural Law > upon conversion to Positive Law..and relative defending rules of Natural Law, converte to Positive ones..) and on basis of relative articles of Constitution, that defends corresponding principles..)/and can be practiced by someone, or involving a relative social, cultural activity.