in mechanical engineering, a device interposed between a source of power and a specific application for the purpose of adapting one to the other. Most mechanical transmissions function as rotary speed changers; the ratio of the output speed to the input speed may be constant (as in a gearbox) or variable. On variable-speed transmissions the speeds may be variable in discrete steps (as on an automobile or some machine-tool drives) or they may be continuously variable within a range. Step-variable transmissions, with some slip, usually employ either gears or chains and provide fixed speed ratios with no slip; stepless transmissions use either belts, chains, or rolling-contact bodies.

A widely used and inexpensive stepless drive consists of a V-belt running on variable-diameter pulleys. The sides of the pulleys are conical on the inside to match the taper of the V-belt, and moving them closer together causes the V-belt to move outward from the centre of the pulley and operate on a larger effective circle; this movement changes the speed ratio. Such drives depend on friction and are subject to slip.

Stepless transmissions employing rolling-contact bodies are known as traction drives. In these transmissions, power is transmitted in a variety of ways that depend on the rolling friction of bodies in the form of cylinders, cones, balls, rollers, and disks.