An example of illegal as an adjective is the phrase an illegal person, who is someone who is still in the United States after their tourist visa has run out.
An example of illegal as an adjective is the phrase an illegal act, which is an act such as armed robbery.
An illegal pass in football.
An illegal operation.
- (chiefly US, sometimes offensive) Being an illegal immigrant; residing in a country illegally.
Illegal immigrant; illegal logger; illegal pilot.