undergraduate vs undergrad

undergraduate and undergrad both are nouns.

Word	Noun	Adjective	Verb	Adverb
undergraduate	Yes	No	No	No
undergrad	Yes	No	No	No

As nouns, undergraduate and undergrad are synonyms defined as:

undergraduate and undergrad: a university student who has not yet received a first degree

undergraduate (noun)	undergrad (noun)
a university student who has not yet received a first degree	a university student who has not yet received a first degree