custom vs Americanism

custom and Americanism both are nouns.

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

As nouns, Americanism is a hyponym of custom; that is, Americanism is a word with a more specific, narrower meaning than custom:

Other hyponyms of custom include Anglicism, Britishism, consuetude, couvade, Germanism, habit, use, hijab, rite, ritual.

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

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