This has been bugging me for a little while now, but I just got around to posting it. My physics teacher said that an acceleration equal to 0 was NOT a constant acceleration as the was no acceleration to speak of. I contended that given a(t) = 0 for all t that a(x) - a(y) = 0 for all x and y (that exist of course) and therefore an acceleration of 0 was indeed constant. My teacher tried to explain it to me several times (which I kept trying to refute) and in the end basically said "deal with it". So, do you consider an acceleration (or velocity or anything else you can think of) of zero to be constant.