Vocabulary - Expression

An expression is a mathematical phrase that can contain numbers, variables, and operations. Examples: $3x + 5$, $n^2 - 2n + 1$, $2a + 3b$.

An expression has a value that depends on the variables. If $x = 4$, then $3x + 5 = 3(4) + 5 = 17$. Change $x$, and the value changes.

Expressions don't claim equality. They're just formulas. When you write $=$, you get an equation.