**Definition:**A frequency domain of a signal represents the description of the signal with respect to its frequency content instead of time. The signal is thus described as a sum of (a possibly infinite number of) sinusoidal functions (e.g., sines or cosines) in different frequencies and phases (or, equivalently, with complex exponentials). The actual description is the phase and amplitude of each sine. Definition: A frequency domain of a signal represents the description of the signal with respect to its frequency content instead of time. The signal is thus described as a sum of (a possibly infinite number of) sinusoidal functions (e.g., sines or cosines) in different frequencies and phases (or, equivalently, with complex exponentials). The actual description is the phase and amplitude of each sine.

**Alternative definition:**

**Synonym:**Frequency resolved instrumentation

**References:**

**Related terms:**Frequency domain near infrared spectroscopy