The waterhole refers to an especially quiet section of the electromagnetic spectrum between 18 and 21 centimeters. Hydroxyl radiates at 18 centimeters, and hydrogen at 21 centimers; those two combined form water, and as water is currently thought to be essential to extraterrestrial life advanced enough to generate radio signals, the waterhole is a frequent target of investigation.

A natural hole or hollow containing water, often in the dry bed of an intermittent river.