The term 'measured by a stopwatch' can be summarized by the word 'timed', which has five letters. Timing is the process of measuring the duration of an event or activity, usually with a stopwatch or other device. It is commonly used in sports, where athletes must complete a certain task within a certain amount of time. In addition, timing is also used in many other fields, such as engineering and manufacturing, to ensure that processes are completed in a timely manner.