undergrad vs senior

undergrad and senior both are nouns.

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

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

Other hyponyms of undergrad include co-ed, college girl, lowerclassman, underclassman.

undergrad (noun)	senior (noun)
a university student who has not yet received a first degree	an undergraduate student during the year preceding graduation
	a person who is older than you are

undergrad (adjective)	senior (adjective)
	older; higher in rank; longer in length of tenure or service
	used of the fourth and final year in United States high school or college
	advanced in years; (`aged' is pronounced as two syllables)