acephaly vs abnormalcy

acephaly and abnormalcy both are nouns.

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

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

acephaly: absence of the head (as in the development of some monsters)
abnormalcy: an abnormal physical condition resulting from defective genes or developmental deficiencies

Other hypernyms of acephaly include abnormality.

acephaly (noun)	abnormalcy (noun)
absence of the head (as in the development of some monsters)	an abnormal physical condition resulting from defective genes or developmental deficiencies