A person one knows (not necessarily a close friend); an acquaintance.
駅で昔の知り合いに偶然会った。
I ran into an old acquaintance at the station by chance.
知り合いを通じてその会社に紹介してもらった。
I was introduced to that company through an acquaintance.
彼女は仕事関係の知り合いが多い。
She has many acquaintances from work.