common denominator A quantity into which all the denominators of a set of fractions may be divided without a remainder. For example, the fractions ^{1}/_{3} and ^{2}/_{5} have a common denominator of 15.
A number that will allow fractions with different denominators to be converted into fractions with the same denominator, so that these fractions can be added or subtracted. The fractions can be expressed as whole numbers divided by the common denominator. Thus, 12 is a common denominator for 1/3 and 1/4, since they can be written as 4/12 and 3/12, respectively. (Seelowest common denominator.)
Note: Figuratively, a common denominator is a common factor in different events: “The common denominator in these crimes is the use of inside knowledge of computer systems.”