forehead vs frontal bone

forehead and frontal bone both are nouns.

Word	Noun	Adjective	Verb	Adverb
forehead	Yes	No	No	No
frontal bone	Yes	No	No	No

As nouns, forehead and frontal bone are synonyms defined as:

forehead and frontal bone: the large cranial bone forming the front part of the cranium: includes the upper part of the orbits

Other synonyms of forehead include os frontale.

forehead (noun)	frontal bone (noun)
the part of the face above the eyes	the large cranial bone forming the front part of the cranium: includes the upper part of the orbits
the large cranial bone forming the front part of the cranium: includes the upper part of the orbits