stop watch vs stopwatch

stop watch and stopwatch both are nouns.

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

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

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

stop watch (noun)	stopwatch (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)