Mathematically speaking, pitch angle would be a measure of angular displacement of the aircraft's longitudinal axis around it's lateral axis from some predefined "datum". On the other hand if you are speaking about rotation, you should use pitch rate, which would be derivative of pitch angle and presents the change of a pitch angle in unit of time. This being said, I agree with Genghis' definition, which is probably a lot easier to understand.
I don't normally pick on these things, but I assume the discussion will go in this direction, so no point in delaying the obvious.
