slang word vs slang expression

slang word and slang expression both are nouns.

Word	Noun	Adjective	Verb	Adverb
slang word	Yes	No	No	No
slang expression	Yes	No	No	No

As nouns, slang word and slang expression are synonyms defined as:

slang word and slang expression: a word or expression that is outside of standard usage and is very informal

Other synonyms of slang word include slang term.

slang word (noun)	slang expression (noun)
a word or expression that is outside of standard usage and is very informal	informal language consisting of words and expressions that are not considered appropriate for formal occasions; often vituperative or vulgar
	a word or expression that is outside of standard usage and is very informal