A means of gaining access to a program or system by bypassing its security controls. Programmers often build back doors into systems under development so that they can fixs bugs. If the back door becomes known to anyone other than the programmer, or if it is not removed before the software is released, it becomes a security risk.
The conflict first manifested itself in the debate over the Clipper chip, a US government proposal for a security chip to be incorporated in communications devices which would provide strong encryption, subject to a back door potentially open to government agencies.