An organ stop (or just stop) is a component of a pipe organ that admits pressurized air (known as wind) to a set of organ pipes. Its name comes from the fact that stops can be used selectively by the organist; some can be "on" (admitting the passage of air to certain pipes), while others can be "off" (stopping the passage of air to certain pipes).

The term can also refer to the control that operates this mechanism, commonly called a stop tab, stop knob, or drawknob.

The term is also sometimes used as a synonym for register, referring to rank(s) of pipes controlled by a single stop. Registration is the art of combining stops.