In math and science, a variable is a symbol (usually a letter) that represents a number we don't know yet or a quantity that can change. In the equation x + 5 = 12, the x is a variable standing in for the unknown number 7. In a science experiment measuring how temperature affects plant growth, temperature is one variable and plant height is another.

Variables are placeholders that let us work with changing or unknown values. When you write n to represent “any number” in a math problem, you're using a variable. A scientist studying how sleep affects test scores treats both sleep hours and test scores as variables because they change from person to person.

Variables make it possible to describe patterns and relationships that stay true even when the specific numbers change. The formula for the area of a rectangle, length × width, uses variables so it works for any rectangle, not just one specific size. In computer programming, variables store information that might change as the program runs, like a player's score in a video game.

Outside of math and science, variable describes anything that changes or is unpredictable. Variable weather means conditions that shift frequently. A teacher might say student effort is the most important variable in academic success.