A stopwatch is a watch with buttons which you press at the beginning and end of an event, so that you can measure exactly how long it takes.
stopwatch in British English
(ˈstɒpˌwɒtʃ)
noun
a type of watch used for timing events, such as sporting events, accurately, having a device for stopping the hand or hands instantly
stopwatch in American English
(ˈstɑpˌwɑtʃ)
noun
a watch with a hand or a digital readout that can be started and stopped instantly so as to indicate seconds or fractions of seconds, as for timing races