A common denominator is a number that can be evenly divided by the denominators (bottom numbers) of two or more fractions. When you need to add or subtract fractions like 1/3 and 1/4, you can't just add them directly because the pieces are different sizes. You need to find a common denominator, a number that both 3 and 4 divide into evenly. In this case, 12 works: it's divisible by both 3 and 4. You can then rewrite 1/3 as 4/12 and 1/4 as 3/12, making them easy to add together.

Outside of math, common denominator means a shared characteristic that connects different things or people. When a teacher needs to explain something to a class with students at different levels, she might look for a common denominator in their experiences, something they all understand. If you're trying to find a game that works for both your five-year-old cousin and your teenage brother, you're searching for a common denominator in what they both enjoy.

The phrase lowest common denominator sometimes describes appealing to the most basic level everyone shares, though this can have a negative connotation, suggesting something overly simplified or watered down to reach the widest possible audience.