John, there really isn't a well-accepted term for the beginning of amplifier clipping. To be meaningful it would have to involve an audible level of distortion, not simply the beginning of the flattening of the top of the sine wave when viewed on an oscilloscope. Various distortion levels have been used in published lab tests to represent "clipping". At one extreme the previous Sound & Vision tests used a 0.3% THD level for their clipping power level, although acknowledging that this was well below actual audibility on music. More commonly(e.g. Home Theater Magazine) a 1.0% level has been used to represent the threshold, although this still wouldn't be audible on much music.
So, it isn't really necessary to consider a term such as "headroom" in determining the maximum audibly clean output available. If well-run lab tests on a particular unit are available, an effective clipping level might be considered to be the point at which distortion exceeded 1.0%.
