old friend; old pal
Refers to a friend from the past, often someone you haven't seen in a long time. Slightly formal and literary compared to casual alternatives like 昔の友達.
The other day, I reunited with an old friend from my university days for the first time in ten years.
He returned to his hometown to visit an old friend.
They met in college and became old friends over the years.
While on a business trip, I met up with an old friend.
I was reunited with an old friend after ten years.