microcephalus vs abnormalcy

microcephalus and abnormalcy both are nouns.

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

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

microcephalus: an abnormally small head and underdeveloped brain
abnormalcy: an abnormal physical condition resulting from defective genes or developmental deficiencies

Other hypernyms of microcephalus include abnormality.

microcephalus (noun)	abnormalcy (noun)
an abnormally small head and underdeveloped brain	an abnormal physical condition resulting from defective genes or developmental deficiencies