A gear is a component within a transmission device that transmits rotational torque by applying a force to the teeth of another gear or device. A gear is different from a pulley in that a gear is a round wheel that has linkages ("teeth" or "cogs") that mesh with other gear teeth, allowing force to be fully transferred without slippage. Depending on their construction and arrangement, geared devices can transmit forces at different speeds, torques, or in a different direction, from the power source.

The most common situation is for a gear to mesh with another gear, but a gear can mesh with any device having compatible teeth, such as linear moving racks.

The gear's most important feature is that gears of unequal sizes (diameters) can be combined to produce a mechanical advantage, so that the rotational speed and torque of the second gear are different from those of the first. In the context of a particular machine, the term "gear" also refers to one particular arrangement of gears among other arrangements (such as "first gear"). Such arrangements are often given as a ratio, using the number of teeth or gear diameter as units. The term "gear" is also used in non-geared devices that perform equivalent tasks:

"...broadly speaking, a gear refers to a ratio of engine shaft speed to driveshaft speed. Although CVTs change this ratio without using a set of planetary gears, they are still described as having low and high "gears" for the sake of convention."

The earliest known reference to a gear was around 50 A.D. by Hero of Alexandria, though the Book of Song suggests that the South Pointing Chariot may have employed differential gears as early as the Zhou Dynasty (1045-256 BC) of China.

The Unicode gear "⚙" is found at u+2699.