stopwatch vs stop watch

stopwatch and stop watch both are nouns.

Word	Noun	Adjective	Verb	Adverb
stopwatch	Yes	No	No	No
stop watch	Yes	No	No	No

As nouns, stopwatch and stop watch are synonyms defined as:

stopwatch and stop watch: a timepiece that can be started or stopped for exact timing (as of a race)

stopwatch (noun)	stop watch (noun)
a timepiece that can be started or stopped for exact timing (as of a race)	a timepiece that can be started or stopped for exact timing (as of a race)