a suffix meaning 'nth-year student' or 'in the nth year' (used with a number, e.g., 'first-year student')
彼は大学三年生で、就職活動を始めた。
He is a third-year university student and has started job hunting.
小学校一年生の教室は明るくてにぎやかだ。
The first-grade classroom in elementary school is bright and lively.
新入社員は社内で『一年生』扱いされることが多い。
New employees are often treated like 'first-year' members within the company.