Trapdoor is a secret undocumented entry point into a 
program used to grant access without normal methods of 
access authentication. 

A trap is a software interrupt, usually the result of an 
error condition

a trap is a device which is used to detect , capture and 
harm to an intruder.

a trap door is a secret entry point into a program that 
allows someone who is aware trap door to gain access 
without going through the usual security access procedures.