Peak clipping is the distortion caused when the gain of an amplifier is increased to a point where the high points, or peaks, of the signal or waveform are cut off at a level where the amplifying circuits are driven beyond their overload point. It is also called 'over-modulation'.
Negative peak clipping is the effect where any swift reductions in the AM wave's amplitude are ‘rounded off’ and the output is distorted.
In diode detector, negative peak clipping is the distortion that occurs in the output of diode detector because of unequal ac and dc load impedances of the diode.
Learn more about the various ways to measure or detect the amplitude (as opposed to the power) of a waveform at St-andrews.ac.uk.