nounchiefly North American
early 17th century: from French illégal or medieval Latin illegalis, from Latin in- 'not' + legalis 'according to the law'
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.