Back door : A door in the back part of a building; hence, an indirect way.

Example:


Taxonomy
Backdoor Trojan, Trusting Trust, attack, compilation, asymmetric, virus, computer, free encyclopedia, AFIPS, computer system, paper Reflections, infected, Induc, trapdoor, source code, machine code, disassembler, IEEE Computer Society.

Defination from the web:
backdoor in a computer system (or cryptosystem or algorithm) is a method of bypassing normal authentication, securing remote access to a ... The backdoor may take the form of an installed program (e.g., Back Orifice), or could be a modification to an existing program or hardware device. ...