Refers to a fellow student or graduate of the same school. Often used in formal or alumni-related contexts.
彼は大学の校友です。
He is a university alumnus.
校友会の集まりに参加した。
I attended a gathering of the alumni association.
alumni association
Compound of 校 (school) and 友 (friend). The reading こうゆう follows standard on'yomi.