timer vs stopwatch

timer and stopwatch both are nouns.

Word	Noun	Adjective	Verb	Adverb
timer	Yes	No	No	No
stopwatch	Yes	No	No	No

As nouns, stopwatch is a hyponym of timer; that is, stopwatch is a word with a more specific, narrower meaning than timer:

timer: a timepiece that measures a time interval and signals its end
stopwatch: a timepiece that can be started or stopped for exact timing (as of a race)

Other hyponyms of timer include chronograph, parking meter, stop watch.

timer (noun)	stopwatch (noun)
a timepiece that measures a time interval and signals its end	a timepiece that can be started or stopped for exact timing (as of a race)
a regulator that activates or deactivates a mechanism at set times
(sports) an official who keeps track of the time elapsed