号码

号码 (hàomǎ) is a noun meaning a number used as an identifier or label — a phone number (电话号码), an ID number (身份证号码), a seat number, a jersey number, a shoe size. Crucially, it is not the number you count or calculate with; that is 数字 (shùzì) or 数 (shù). 号码 always tags or identifies a specific item within a series, the way English 'number' works in 'phone number' or 'room number' but not in 'the number three.' So you dial a 号码 and wear a jersey 号码, but you do math with 数字.

This distinction trips up English speakers who use one word 'number' for both. Register is neutral and everyday. Compare 数字 (a numeral/figure for counting or data) and 编号 (a serial/reference number, more formal, for cataloguing).