A stopwatch is a hand-held timepiece designed to measure the time between its activation and deactivation. A large digital version of a stopwatch, such as in a gymnasium, that can be viewed from a distance, is called a stopwatch. In manual timing, the clock is started and stopped by a person pressing the button. In the fully automatic time, the start and stop are automatically triggered by the sensor.

Traditionally, the timing function is controlled by two buttons on the housing. Press the button at the top to start the timer, press the button again to stop the timer and display the elapsed time. Then, press the second button to reset the stopwatch to zero. The second button is also used to record the split time or lap time. When the watch is running, if the "split time" button is pressed, the display will freeze, then start, and then freeze again to read the elapsed time at this point, but the watch mechanism continues to run to record the total elapsed time. Press the split button again to restore the total time displayed on the watch.

The mechanical stopwatch is driven by the spring, and the spring must be tightened regularly by turning the knurled knob at the top of the watch.

The digital electronic stopwatch is much more accurate than the mechanical one because of its crystal oscillator timing element. Because they contain microchips, they often also contain date and time functions. Some may have connectors for external sensors that allow stopwatches to be triggered by external events, so the elapsed time can be measured more accurately than by pressing a button with a finger. Stopwatches are usually measured in 1 / 100 of a second, not in milliseconds. The first digital timer for organized sports was digitimer, which was developed by Cox electronic systems (1971) in Salt Lake City, Utah. It uses digital tube readings and provides a resolution of 1 / 1000 second. It was originally used for skiing, but was later used in the Universiade in Moscow, Russia, NCAA in the United States and the Olympic Games.

The device can be used when the time period must be accurately measured and complications are minimal. Laboratory experiments and sprints are good examples.

Stopwatch function also exists as an additional function of many electronic devices (such as watches, mobile phones, portable music players and computers).

In most scientific experiments, researchers usually use Si or Si in any experiment. For stopwatches, the usual units of time to observe stopwatches are minutes, seconds, and hundredths of a second.