Stopwatch (noun)

1. A watch with a stop function that can be started, stopped, and reset at will.
2. A watch that can measure the time elapsed from when it is started until it is stopped and can be reset to zero.

Origin:

From stop + watch.

Examples:

1. The coach used a stopwatch to time the runner's laps.
2. He used a stopwatch to measure how long it took to complete the task.
3. The stopwatch was a useful tool for timing races.
4. The stopwatch was set to measure the time of the event.
5. The stopwatch was accurate to within a hundredth of a second.