What is a sound of single frequency called?

A sound of a single frequency is called a pure tone or a sine wave. Pure tones are characterized by having a single frequency and are often used as reference signals in various fields such as acoustics, audio engineering, and physics. They have a simple, smooth waveform with no harmonics or overtones. Pure tones are essential in auditory research, music theory, and sound analysis due to their simplicity and well-defined frequency content.