咱(zán) - we, us (including you) in Mandarin

咱 (zán) means we, but with a promise built in: the person you are talking to is included. 我们 can go either way, so 我们去吃饭 might mean the rest of us are off to eat without you. 咱去吃饭 never can.

It belongs to northern Mandarin and to relaxed speech. In Beijing you hear it constantly; in Taiwan and much of the south you hear 我们 instead. The fuller form 咱们 is more common in writing and in careful speech, while the bare 咱 turns up in quick colloquial lines like 咱走吧 and in fixed pairs such as 咱俩 and 咱家.

The trap for English speakers is not the meaning but the register. 咱 is warm and familiar, the pronoun of family and mates. Using it in a formal email or with a stranger sounds oddly matey, so stick to 我们 until you know the room.