onanism vs jerking off

onanism and jerking off both are nouns.

Word	Noun	Adjective	Verb	Adverb
onanism	Yes	No	No	No
jerking off	Yes	No	No	No

As nouns, jerking off is a hyponym of onanism; that is, jerking off is a word with a more specific, narrower meaning than onanism:

onanism: manual stimulation of the genital organs (of yourself or another) for sexual pleasure
jerking off: slang for masturbation

Other hyponyms of onanism include self-abuse, self-stimulation, frottage, hand job, jacking off, wank.

onanism (noun)	jerking off (noun)
manual stimulation of the genital organs (of yourself or another) for sexual pleasure	slang for masturbation
a method of birth control in which coitus is initiated but the penis is deliberately withdrawn before ejaculation