A watch made for timing things exactly: you press once to start it, once to stop it, and it shows how long something took, usually down to hundredths of a second. It is the coach's tool at the side of the running track, and also the science teacher's, the nurse's counting a pulse, the factory manager's timing how long a job takes. These days most people use the stopwatch app on their phone, but the word did not change — you still 'start the stopwatch', 'stop the stopwatch', 'time someone with a stopwatch'. There is a verb too, to stopwatch something, but it is rare; normally you say 'time it'. The phrase 'stopwatch precision' or 'to the stopwatch' means measured very exactly.