noun
acquaintance; person one knows
Refers to someone one is acquainted with, but not necessarily a close friend. Often used in formal or written contexts.
彼は仕事上の知己に過ぎない。
He is merely a business acquaintance.
noun
A closer, more trusted friend than a mere acquaintance. This usage is somewhat literary and less common in casual speech.
彼は数少ない私の知己の一人だ。
He is one of my few close friends.