In digital recording no signal exists beyond 0 dB, however, in analog recording peaks can and do exceed this point (e.g. +1dB). When the analog signal (original narration or tape transfer) exceeds 0dB, the signal peak in the digital audio reaches and stays at 0dB. If this continues for more than a short period of time ((approximately 10 samples), the result is "clipping" which results in distortion. Distortion resulting from high input levels in a digital production environment is much more noticeable and unpleasant than distortion resulting from high input levels in an analog production environment. It is essential to control digital recording levels so that the peaks do not exceed 0 dB.
Amplifier distortion occurring when a high energy wave form (a very loud sound resulting in a large output) is input into an amplifier and the amplifier is unable to fully reproduce it due to power supply limitations or amplifier design limitations resulting in the audio output waves being cut off (the rounded tops sliced off resulting in short waves with flat tops).
Amplifiers do this when they are being over driven, and are close to their working limit. The word refers to the top and bottom of the signal being clipped off, and this causes distortion, which can very easily damage speakers. All amplifiers have a light on the front panel to indicate this is happening, but the effect is very audible.