TRAP 1. n. A program interrupt, usually used specifically to refer to an interrupt caused by some illegal action taking place in the user program. In most cases the system monitor performs some action related to the nature of the illegality, then returns control to the program. See UUO. 2. v. To cause a trap. "These instructions trap to the monitor." Also used transitively to indicate the cause of the trap. "The monitor traps all input/output instructions."