An old friend; someone you have been close to for a long time.
昨日、旧友と久しぶりに再会して飲んだ。
Yesterday I met up with an old friend after a long time and had drinks.
旧友の成功を心から喜んだ。
I was genuinely happy for my old friend's success.
彼とは旧友だから、なんでも話せる。
He's an old friend, so I can tell him anything.