nine·ty / ˈnīntē/ • n. (pl. -ties) equivalent to the product of nine and ten; ten less than one hundred; 90: ninety acres of soybeans will be harvested. (Roman numeral: xc or XC.) ∎ (nineties) the numbers from 90 to 99, esp. the years of a century or of a person's life: art in the nineties. ∎ ninety years old: she is nearly ninety. ∎ ninety miles an hour: we passed the junction doing about ninety.DERIVATIVES: nine·ti·eth / -tēi[unvoicedth]/ ordinal number.nine·ty·fold / -ˌfōld/ adj. & adv.