A somewhat literary or formal term for a person one has known for a long time. In everyday speech, 昔の友達 or 古い友人 are more common.
He is an old friend from my university days.
I ran into an old acquaintance.
Compound of 旧 (old, former) and 識 (acquaintance, knowledge). The exact historical derivation is uncertain, but the word follows a standard Sino-Japanese pattern for formal nouns.