The only definition I can give of an "assault" rifle (or any other tool, for that matter) is if it's being used to assault somebody. So an M4A1 used by a soldier in the sandbox for example, is an assault rifle (or more accurately, "defense rifle" under the present ROE). The mechanically identical rifle used by a competitor in High Power competitions is a "target rifle". The same rifle used to kill feral hogs is a "hunting rifle". The furniture of the rifle itself is just a detail.
I agree with Lorax that anti-gun media types have made the term popular as meaning "bad and scary", in a stupid display of fuzzy speech that can lead to fuzzy thinking. To me, words have specific meanings, and I want to use them accurately.
Parker