减速器是原动机和工作机之间独立的闭式传动装置，用来降低转速和增大转矩以满足各种工作机械的需要。

按传动和结构特点划分，减速器种类有齿轮减速器（Gear reducer）、蜗杆减速器(Worm wheel reducer）、蜗杆齿轮减速器及齿轮-蜗杆减速器、行星齿轮减速器(Planetary gears)、摆线针轮减速器（Cycloid reducer）和谐波齿轮减速器(Harmonic drive)等。

A reduction drive is a mechanical device to shift rotational speed. A planetary reduction drive is a small-scale version using ball bearings in an epicyclic arrangement instead of toothed gears.

Reduction drives are used in engines of all kinds, to increase the amount of torque per revolution of a shaft: the gearbox of any car is a ubiquitous example of a reduction drive. Common household uses are washing machines, food blenders and window-winders.