Used for non-stationary signals whose frequency content changes over time, such as a musical note that slides in pitch.

A measure often used in damage identification or onset detection to describe the "center of mass" of a signal's frequency content. Common Applications Frequency Content - an overview | ScienceDirect Topics

The primary mathematical operation used to decompose a signal into its constituent sine waves. While a spectrum analyzer can visualize this in real-time, the Fast Fourier Transform (FFT) is the standard algorithm for digital computation.