periodontia vs odontology

periodontia and odontology both are nouns.

Word	Noun	Adjective	Verb	Adverb
periodontia	Yes	No	No	No
odontology	Yes	No	No	No

As nouns, odontology is a hypernym of periodontia; that is, odontology is a word with a broader meaning than periodontia:

periodontia: the branch of dentistry dealing with diseases of the gums and other structures around the teeth
odontology: the branch of medicine dealing with the anatomy and development and diseases of the teeth

Other hypernyms of periodontia include dental medicine, dentistry.

periodontia (noun)	odontology (noun)
the branch of dentistry dealing with diseases of the gums and other structures around the teeth	the branch of medicine dealing with the anatomy and development and diseases of the teeth