An action, datum or data that is not legal. (Not legal for the computer's internal rules, not in the real world!)
Illegality is not transitient or temporary, it is permanent. (Re)Attempting an illegal action or (re)testing illegal datum or data should result in failure.
The famous error message on Windows computers is something like "This program has performed an illegal operation and will be shut down", which isn't a very meaningful error message to the majority of end users. Such messages are surprisingly common when one system wraps (or emulates or similiar) another and the wrapper doesn't want to go to the effort of wrapping each error message. A better message would say that there is a bug in the software and the program can't continue running.