A stop clock, also known as a stopwatch, is a laboratory tool used to measure time intervals with high precision. It consists of a clock mechanism housed in a small, handheld device that can be started, stopped, and reset using buttons or other controls.

Some stop clocks may also have additional features, such as the ability to record multiple time intervals or to display elapsed time in different units (e.g. seconds, minutes, hours).

Stop clocks are commonly used in scientific experiments and other applications where precise timing is required, such as sports training, cooking, and manufacturing. They are also used in educational settings to teach concepts such as measurement, timing, and experimental design.