common denominator

A common denominator is a shared multiple of the denominators in a set of fractions, allowing them to be added or compared: to add 1/3 and 1/4, you convert both to twelfths.

Figuratively, a common denominator is any shared characteristic among a group: The common denominator in all these cases is poverty.

The phrase lowest common denominator refers mathematically to the smallest such shared multiple, but in general use describes the most basic or unsophisticated level of taste or understanding: Movies that appeal to the lowest common denominator.