In communications, noise spectral density, noise power density, noise power spectral density, or simply noise density is the power spectral density of noise or the noise power per unit of bandwidth.

In statistical signal processing, the goal of spectral density estimation or simply spectral estimation is to estimate the spectral density (also known as the power spectral density) of a signal from a sequence of time samples of the signal.

In spectroscopy, spectral flux density is the quantity that describes the rate at which energy is transferred by electromagnetic radiation through a real or virtual surface, per unit surface area and per unit wavelength (or, equivalently, per unit frequency).