Backdoor
From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
A backdoor, in computer terminology, is a programmed exploit that permits remote access into a computer system, usually at a level of total control.
Backdoors can be compared to the keys for the locks on the doors of your home. A burglar could attempt to smash his way into your home, which is the way that some virus attacks work. Or, a smart burglar could obtain copies of your keys, and use the key to open the way into your house as you would.
Backdoors were installed by a Cylon agent in Gaius Baltar's Command Navigation Program which permit the Cylons to remotely deactivate ships during the Fall of the Twelve Colonies, allowing them an easy victory against their creators on the Twelve Colonies.