Formal:
* 朋友 (péngyou) - This is the most common and general term for "friend". It can be used in any situation.
* 友人 (yǒurén) - This is a slightly more formal term for "friend". It's often used in writing or when addressing someone older or of higher social status.
Informal:
* 伙伴 (huǒbàn) - This means "partner" or "comrade" and can be used for close friends.
* 哥们 (gēmen) - This is a very informal term for "buddy" or "mate" and is typically used among male friends.
* 姐妹 (jiěmèi) - This means "sisters" and is used for close female friends.
Other variations:
* 知己 (zhījǐ) - This means "soulmate" or "intimate friend".
* 死党 (sǐdǎng) - This is a very informal term for "best friend".
The choice of word depends on the context and your relationship with the person.
For example, you would use 朋友 (péngyou) to introduce someone as your friend, while you might use 伙伴 (huǒbàn) or 哥们 (gēmen) to refer to a close friend in a casual setting.