An operation that is not authorized or understood. An "illegal operation" error message typically means that the computer has been directed to execute an invalid instruction and has stopped or has terminated the offending application (see abend). However, programmers may choose to display such a message at certain locations in their code as a safety check, trusting that their logic is sound and the computer should never arrive at those points. If it does, the program halts, and the message is displayed. Such messages may have an error number that refers to a location in the code.