unwelcome guest vs crasher

unwelcome guest and crasher both are nouns.

Word	Noun	Adjective	Verb	Adverb
unwelcome guest	Yes	No	No	No
crasher	Yes	No	No	No

As nouns, unwelcome guest and crasher are synonyms defined as:

unwelcome guest and crasher: someone who gets in (to a party) without an invitation or without paying

Other synonyms of unwelcome guest include gatecrasher.

unwelcome guest (noun)	crasher (noun)
someone who gets in (to a party) without an invitation or without paying	someone who gets in (to a party) without an invitation or without paying