座位(zuò wèi) - seat, place to sit in Mandarin

座位 (zuò wèi) is a seat: the numbered place on a train, in a cinema, on a plane, in a classroom. The measure word is 个. It is the noun you want when the point is the allocated spot rather than the furniture.

Everyday phrases cluster around it. 空座位 is a free seat, 让座位 is to give up your seat, 找座位 is to look for one, and 对号入座 is to sit in the seat matching your number. In casual northern speech people often shorten it to 位子 (wèi zi), which means the same thing.

The trap is confusing the seat with the sitting and with the furniture. 坐 (zuò) is the verb to sit, and it is the same syllable and tone as 座, which makes it easy to write the wrong character. 椅子 is a chair as an object you could carry out of the room; 座位 is a place in a seating plan. A stadium has 座位 rather than 椅子, even though every one of them has a chair in it.