acquaintance; person one knows
Refers to someone one is acquainted with, but not necessarily a close friend. Often used in formal or written contexts.
He is merely a business acquaintance.
A closer, more trusted friend than a mere acquaintance. This usage is somewhat literary and less common in casual speech.
He is one of my few close friends.
While studying abroad, I made the acquaintance of many notable people.
I quickly became close friends with him.
I have an old acquaintance in this town.
友人 is a general, neutral word for 'friend', whereas 知己 in its second sense implies a deeper, more literary bond of mutual understanding.
親友 is the common word for a 'best friend', while 知己 conveys a similar closeness but with an old-fashioned, bookish tone.
自己 (meaning 'oneself') shares the kanji 己 with 知己 but has an unrelated meaning of self rather than acquaintance.
From Chinese 知己 (zhījǐ), literally 'knowing oneself', extended to mean a person who truly understands one, hence a close friend or acquaintance.