dartboard vs dart board

dartboard and dart board both are nouns.

Word	Noun	Adjective	Verb	Adverb
dartboard	Yes	No	No	No
dart board	Yes	No	No	No

As nouns, dartboard and dart board are synonyms defined as:

dartboard and dart board: a circular board of wood or cork used as the target in the game of darts

dartboard (noun)	dart board (noun)
a circular board of wood or cork used as the target in the game of darts	a circular board of wood or cork used as the target in the game of darts