Generally the highest point. In audio this refers in various ways to the maximum audio signal. A sine wave has two peaks per cycle, which represent the maximum or peak amplitude or voltage (one is maximum in the positive direction while the other is maximum in the negative). Complex musical signals have peaks, which represent the loudest sections or moments. Transients in musical material are also referred to as peaks, though they are really a specific type of peak that has a very short duration of time. On a waveform display such as an oscilloscope audio peaks often look like the peaks of a mountain.
Peaking is a word that is sometimes used to describe audio that has gone beyond some specified reference. For example, when the peak LED illuminates on a mixer or recorder the signal can be said to be peaking. This is closely associated with overloading, distortion, and clipping.