Short name for an early sound modification device (the full name is Leslie Rotating Speaker), invented in the late 1940's by Don Leslie. The leslie is often used together with some sort of electric organ or derivative thereof (like a Moog), and was extremely popular in the 70's and 80's rock scene, and has become rather fashionable with retro and indie bands of the late nineties.
The principle behind the leslie is easy - the sound is output through one or more horn drivers which rotate horizontally around a pivot point. This affects the sound in several ways: All these effects coupled with some chorus from the organ itself and variable rotor speed produce a complex, rich sound.