first-year vs freshman

first-year is not a noun while freshman is a noun.

Word	Noun	Adjective	Verb	Adverb
first-year	No	Yes	No	No
freshman	Yes	Yes	No	No

As adjectives, first-year and freshman are synonyms defined as:

first-year and freshman: used of a person in the first year of an experience (especially in United States high school or college)

first-year (noun)	freshman (noun)
	a first-year undergraduate
	any new participant in some activity

first-year (adjective)	freshman (adjective)
used of a person in the first year of an experience (especially in United States high school or college)	used of a person in the first year of an experience (especially in United States high school or college)