‘序数’ (xù shù) literally means ‘sequence number’ — ‘序’ (xù) refers to order, sequence, or arrangement, while ‘数’ (shù) means ‘number’. Together, they form the technical term for ‘ordinal number’, i.e., numbers that indicate position or rank in a series (e.g., first, second, third), as opposed to ‘基数’ (jī shù), which are cardinal numbers used for counting (e.g., one, two, three). In Chinese, ordinal numbers are typically formed by adding 第 (dì) before a cardinal number: 第一 (dì yī, ‘first’), 第二 (dì èr, ‘second’), etc.

‘序数’ is commonly used in academic, mathematical, linguistic, and instructional contexts — for example, when teaching grammar (e.g., ‘序数词’ means ‘ordinal numeral’), discussing rankings, dates (e.g., 第25届, ‘the 25th edition’), or floor numbers in buildings. It’s a formal, neutral-term noun rarely used in casual speech, where speakers simply say 第… instead of naming the category itself.