A watch that can be instantly started and stopped by pushing a button and used to measure an exact duration of time. (noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "stopwatch" in a sentence
"But the scoreboard or stopwatch is supposed to be reliable."
"The Segway's mileage and maintenance is tracked by a small key, similar in appearance to a stopwatch, which is also needed to start the Segway."
"A stopwatch was the first, but a more accurate solution was a bicycle wheel, which was mounted to the car's rear."