or·di·nal / ˈôrdn-əl/ • n. 1. short for ordinal number.2. Christian Church, chiefly hist. a service book, esp. one with the forms of service used at ordinations.• adj. of or relating to a thing's position in a series: ordinal position of birth. ∎ of or relating to an ordinal number. ∎ Biol. of or relating to a taxonomic order.