The "height" of a waveform. For audio, greater amplitudes correspond to greater signal strengths or loudness (volume). In technical terms, amplitude is the magnitude, or level of variation away from the zero value, in an electrical signal or acoustical sound wave (i.e., the range of variations in voltage levels or air displacement caused by that periodic waveform). Greater amplitudes represent more intense electrical signal, pressure variation, or air displacement.
In other words, for music and audio, louder sounds have relatively greater amplitudes. (The vibrations produced by a louder version of a sound wave actually cause air molecules to oscillate back and forth through greater distances.) Acoustic amplitudes (sound pressure levels) are measured in decibels, or dB (see definition in Locator!). To be meaningful, any figures cited for amplitude must also specify the measurement method, peak, rms, instantaneous, maximum, etc.
Crank it up Tough Guy, I want to feel my pants move! But beware: too much Sound Pressure Level (SPL, related to the amplitude of acoustic waveforms) may damage valuable things hanging off the sides of your head.
In the electronic world, amplitudes beyond the capabilities of the equipment result in "clipping" (a distortion of the waveform, where the peaks are "cut off", due a given component's inability to reproduce the entire excursion of the input waveform).
(Check out the "Picture" Media icons for a Sound pressure level scale, and a view of a clipped waveform.)
