Americanism vs custom

Americanism and custom both are nouns.

Word	Noun	Adjective	Verb	Adverb
Americanism	Yes	No	No	No
custom	Yes	Yes	No	No

As nouns, custom is a hypernym of Americanism; that is, custom is a word with a broader meaning than Americanism:

Other hypernyms of Americanism include usage, usance.

Americanism (noun)	custom (noun)
a custom that is peculiar to the United States or its citizens	accepted or habitual practice
loyalty to the United States and its institutions	habitual patronage
an expression that is characteristic of English as spoken by Americans	a specific practice of long standing
	money collected under a tariff

Americanism (adjective)	custom (adjective)
	made according to the specifications of an individual