A representation (of a sound signal, for instance) as some sort of intensity as a function of time and frequency. In Praat, we have the Spectrogram, which is acoustic energy density as a function of time in seconds and frequency in Hz, and the Cochleagram, which is basilar membrane excitation as a function of time in seconds and frequency in Bark.

For tutorial information, see Intro 3.1. Viewing a spectrogram.