an 'illegal alien/immigrant' [=a foreign person who is living in a country without having official permission to live there]
The campaign was accused of many fund-raising 'illegalities'. [=illegal activities]
In this state, it is 'illegal' for anyone under the age of 21 to drink alcohol.
(..) but... but that doesn't matter... 'cause, get a load of this, all right? If you get stopped by a cop in Amsterdam... it's illegal for them to search you. I mean, that's a right the cops in Amsterdam don't have. Oh, man, I'm going. That's all there is to it. (..)1994 Pulp Fiction