megacephaly vs abnormalcy

megacephaly and abnormalcy both are nouns.

Word	Noun	Adjective	Verb	Adverb
megacephaly	Yes	No	No	No
abnormalcy	Yes	No	No	No

As nouns, abnormalcy is a hypernym of megacephaly; that is, abnormalcy is a word with a broader meaning than megacephaly:

megacephaly: an abnormally large head; differs from hydrocephalus because there is no increased intracranial pressure and the overgrowth is symmetrical
abnormalcy: an abnormal physical condition resulting from defective genes or developmental deficiencies

Other hypernyms of megacephaly include abnormality.

megacephaly (noun)	abnormalcy (noun)
an abnormally large head; differs from hydrocephalus because there is no increased intracranial pressure and the overgrowth is symmetrical	an abnormal physical condition resulting from defective genes or developmental deficiencies