Illegal means forbidden by law. Illicit is wider — it covers anything forbidden by law, rule or custom, and often carries a hint of secrecy or disapproval. All illegal acts are illicit; not all illicit ones are illegal. An affair can be illicit without breaking any law.
The core difference
Illegal is a legal judgement: a statute forbids it. Illicit is broader and softer at the edges — it can mean against the law, but it also covers what is against the rules, against convention, or simply not openly done.
- An illegal parking spot. — a law says no
- An illicit affair. — no law broken, but concealed and disapproved of
- The illicit drug trade. — both at once
The test
Ask whether a court could rule on it. If the answer is yes, illegal is precise. If the objection is moral, social or institutional rather than legal, illicit is the word — and it brings a whiff of secrecy that illegal does not.
The neighbours
English has several words in this space and they are not interchangeable:
- illegal — against the law
- unlawful — not authorised by law; often used where something is void or improper rather than criminal
- illicit — against law, rule or custom, usually covertly
- illegitimate — not properly authorised or justified; also a historical term for children born outside marriage
In law, unlawful and illegal are often distinguished: an unlawful contract is unenforceable, while an illegal act is punishable. Outside law the two are used loosely.
Elicit, which is a different word entirely
Elicit is a verb meaning to draw something out — "to elicit a response". It sounds identical to illicit and means something unrelated, which makes it one of the commonest homophone errors in written English.
