frontal bone vs forehead

frontal bone and forehead both are nouns.

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

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

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

Other synonyms of frontal bone include os frontale.

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