A piece of code built into software which allows it's creator entry. This type of system is usually unneccecary, but has two main reasons.
1. To automatically create an account with full priviledges that is hidden from the user(s) of the program, so any time they get locked out the creator can log in and solve the prblem for them (a maintenance account)
2. as above, but with ulterior motives. See spyware
3. As above, but because the software is not designed for used by it's creator.
Particularly common in movies about or involving computer hacking.
1. "The user had forgotten his password, so I had to use the backdoor and create him a clean account."
2. "The moron downloaded the program and now I'm watching his every move through the backdoor!"
3. "I was designing a new piece of control software for NASA, but I threw in an emergency backdoor as a precaution."