齿轮（Gear或cogwheel）是轮缘上有齿能连续啮合传递运动和动力的机械组件，齿轮依靠齿的啮合传递扭矩的轮状机械零件。齿轮通过与其它齿状机械零件（如另一齿轮、齿条、蜗杆）传动，可实现改变转速与扭矩、改变运动方向和改变运动形式等功能。由于传动效率高、传动比准确、功率范围大等优点，齿轮机构在工业产品中广泛应用，其设计与制造水平会直接影响到工业产品的品质。

「齿轮轮齿相互扣住齿轮会带动另一个齿轮转动来发送动力。将两个齿轮分开，也可以应用链条、履带、皮带来带动两边的齿轮而发送动力。」齿轮一般由轮齿、齿槽、端面、法面、齿顶圆、齿根圆、基圆和分度圆组成。

A gear or cogwheel is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part to transmit torque. Geared devices can change the speed, torque, and direction of a power source. Gears almost always produce a change in torque, creating a mechanical advantage, through their gear ratio, and thus may be considered a simple machine. The teeth on the two meshing gears all have the same shape. Two or more meshing gears, working in a sequence, are called a gear train or a transmission. A gear can mesh with a linear toothed part, called a rack, thereby producing translation instead of rotation.