the.com/common denominator

means A shared quality, interest, or standard that applies to every member of a group, making comparison or agreement possible — or, in math, the shared multiple that lets fractions with different bottoms be added or compared.

from Straight from arithmetic: 'denominator' comes from Latin denominare, 'to name' — the number below the line that names what kind of fraction you're dealing with (halves, thirds, etc). When fractions have different denominators, you can't easily compare or add them, so you find a 'common' one — a shared multiple both can convert to. The phrase leapt into everyday English by the 19th century as a metaphor for whatever trait or baseline a group of people or things shares, often with a faint whiff of judgment, as in 'lowest common denominator,' used to describe the least demanding or most crudely appealing shared trait.