定时器是一种特殊类型的时间间隔测量的时钟。定时器发出提示的声音代表时间间隔已过期。

定时器有两大类：硬件和软件定时器。

机械计时器使用钟表来测量时间。

机械定时器的最简单和最古老的类型是沙漏 ，其中砂排水管通过一个狭窄的开口从一个室到另一个固定量以测量的时间间隔。

一种凸轮计时器采用了小型同步交流电机转动凸轮对梳开关触点。该计时器的最常见的应用是现在在洗衣机、干燥器和洗碗机。这种类型的定时器通常具有齿轮系和凸轮之间的摩擦离合器，从而使凸轮可转向重置时间。

A timer is a specialized type of clock for measuring time intervals. By function timers can be categorized to two main types. A timer which counts upwards from zero for measuring elapsed time is often called a stopwatch; a device which counts down from a specified time interval is more usually called a timer or a countdown timer. A simple example for this type is an hourglass. By working method timers have two main groups: Hardware and Software timers.