mastoid bone vs outgrowth

mastoid bone and outgrowth both are nouns.

Word	Noun	Adjective	Verb	Adverb
mastoid bone	Yes	No	No	No
outgrowth	Yes	No	No	No

As nouns, outgrowth is a hypernym of mastoid bone; that is, outgrowth is a word with a broader meaning than mastoid bone:

mastoid bone: process of the temporal bone behind the ear at the base of the skull
outgrowth: a natural prolongation or projection from a part of an organism either animal or plant

Other hypernyms of mastoid bone include appendage, process.

mastoid bone (noun)	outgrowth (noun)
process of the temporal bone behind the ear at the base of the skull	a natural prolongation or projection from a part of an organism either animal or plant
	the gradual beginning or coming forth
	a natural consequence of development