(1) (noun) If a compressor or limiter is subjected to a sudden large input signal level, its attack time may not be fast enough to prevent the output from being momentarily too high. This initial excessive level is called overshoot, and its severity depends on the speed of the device.
(2) (verb) Imperfect transient response in an audio device will result in the waveform going past the desired value on fast signal transitions, as is frequently seen on square waves and the impulse responses of CD players.