stopwatch vs timer

stopwatch and timer both are nouns.

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

As nouns, timer is a hypernym of stopwatch; that is, timer is a word with a broader meaning than stopwatch:

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

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