n. 1. a ratio of two integers, or any number that can be expressed as such a ratio, m/n, where m is not a multiple of n, and n is not zero or one. The rules for addition and multiplication of fractions are from which the rules for subtraction, division, and inversion can readily be derived. See decimal fraction, vulgar fraction, proper fraction. 2. any ratio of one quantity or expression (the numerator) to another non-zero quantity or expression (the denominator).