backdoor definition

Definition of backdoor : noun

1. Any method by which authorized and unauthorized users are able to bypass normal security measures and gain access to an otherwise inaccessible system, especially an undocumented or publicly unknown method to access a computer system undetected or to break a cypher used to encode messages.
   The New York Times implied that the NSA, as part of a $250-million decade-long covert operation, installed backdoors into the algorithm of a number of encryption systems used by millions of people around the world.