contrary to or forbidden by law, especially criminal law:

違法の, 不法の, 非合法の(↔ legal)

e.g. illegal drugs.

a person present in a country without official authorization.

｟米話｠ 不法入国者

early 17th century: from French illégal or medieval Latin illegalis, from Latin in- ‘not’ + legalis ‘according to the law’.