A loudspeaker which rotates, or more commonly, a (stationary) loudspeaker inside an enclosure which rotates. Typically, the enclosure consists of a cylinder, with a hole cut at some rotational position, and closed at the top and bottom. It may also use a rotating horn. The loudspeaker itself fires into the cylinder from the top or bottom, and sound emerges from the rotating hole. A motor rotates the enclosure at a fairly slow speed, ranging from 10-150 revolutions per minutes typically. Usually, this will all be encased within a stationary cabinet.
In the canonical Leslie design often used with Hammond organs, the incoming signal is split by a 2-way crossover to drive a tweeter and woofer. The woofer fires into a rotating cylinder; the tweeter is a "compression driver" that fires into the base of a T-shaped rotating horn. (The sound comes out of one arm of the T; the other side is plugged and is there only for mechanical balance.) Both rotating assemblies are driven by a motor which can be changed between two preset speeds via a switch. Because the rotating assemblies have angular momentum, speed changes are not instantaneous. The use of the two speeds and the accelerating/decelerating effects are part of the canon of Hammond organ playing, particularly as used by jazz, soul, and progressive rock performers.
A rotating speaker produces a sound that might be described as "choir-like" or "wobbly", depending on the motor speed. As the opening in the enclosure rotates, it alternately moves towards and away from the listener, producing a degree of frequency modulation via the Doppler effect. It also produces an amplitude modulation effect, as well as variation in timbre, as the sound first comes directly at the listener, and then indirectly via reflection when the opening faces away from the listener. The effect is an excellent way of adding interest to relatively static sounds such as pure electronic tones, which is why it became popular for use with the Hammond organ. However, it has been used with other instruments. Jimi Hendrix often experimented with running his guitar through a Leslie, and Tony Banks used to run all of his instruments through a Leslie. It is a surprisingly difficult effect to emulate electronically with accuracy, which is one reason that actual rotating speakers are still manufactured; not only is the Leslie company still in existence, but they have a competitor in the form of Motion Sound.