I'm not sure the term "backdoor" is that well defined. In Ken Thompson's "Reflections On Trusting Trust" Turing award lecture from 1984, he talked about "troja

I'm not sure the term "backdoor" is that well defined. In Ken Thompson's "Reflections On Trusting Trust" Turing award lecture from 1984, he talked about "trojan horses" being inserted into the compiler in such a way that any code run through the compiler results in a program that has behaviour which is unable to be determined from examining the source code. They are "compiled into the build" by a malicious _compiler_, not from malicious bugs introduced into the source code. It's quite obvious that such behaviours could easily be things which deserve the name "backdoor". (It's only short, and _well_ worth the time needed to read it if you haven't come across it before: https://www.archive.ece.cmu.edu/~ganger/712.fall02/papers/p761-thompson.pdf https://www.archive.ece.cmu.edu/~ganger/712.fall02/papers/p7... )