1540s, in mathematics, "that term of a fraction which indicates the value of the fractional unit" (commonly the number written below the numerator or dividend), from Medieval Latin denominator, agent noun from past-participle stem of denominare "to name," from de- "completely" (see de-) + nominare "to name," from nomen "name" (from PIE root *no-men- "name"). As "one who or that which gives a name," 1570s.
