a number that refers to the position of something in a series, for example ‘first’, ‘second’, etc. 序数词（如第一、第二等） Word OriginMiddle English (referring to a service book in the Christian church): the noun from medieval Latin ordinale (neuter); the adjective from late Latin ordinalis ‘relating to order in a series’, from Latin ordo, ordin- ‘row, series, rank’.