shepherd dog vs sheepdog

shepherd dog and sheepdog both are nouns.

Word	Noun	Adjective	Verb	Adverb
shepherd dog	Yes	No	No	No
sheepdog	Yes	No	No	No

As nouns, shepherd dog and sheepdog are synonyms defined as:

shepherd dog and sheepdog: any of various usually long-haired breeds of dog reared to herd and guard sheep

Other synonyms of shepherd dog include sheep dog.

shepherd dog (noun)	sheepdog (noun)
any of various usually long-haired breeds of dog reared to herd and guard sheep	any of various usually long-haired breeds of dog reared to herd and guard sheep