What is the name for a number that can be expressed as a ratio of two integers?

A rational number is a number that can be expressed as a ratio of two integers, with a nonzero denominator.

Examples include 1/2, -3/4, 5 and 0. Whole numbers are rational because they can be written over 1: for example, 5 equals 5/1. Rational numbers may have terminating decimal expansions, such as 0.25, or repeating decimal expansions, such as 0.333…, which equals 1/3.

The word comes from the idea of a ratio, not from rationality in the everyday sense. The key condition is that both parts of the fraction are integers and the denominator cannot be zero. Numbers such as pi and the square root of 2 are irrational because they cannot be represented exactly as a ratio of integers. Rational and irrational numbers together make up the real numbers, so every rational number is real, but not every real number is rational.