An expression is a group of connected numbers, operations such as addition and multiplication, and unknown variables represented by letters of the alphabet. It differs from an equation in that it does not contain an equal sign and cannot be solved.

The three most common types of expressions are arithmetic expressions, which contain one variable, numbers and simple operations; polynomial expressions, which contain more than one variable; and algebraic expressions, which can contain all of the above in addition to exponents, or powers.

To simplify an expression, a person must combine like terms. For instance, in the expression "4x+3y+12x+4+6y+10," the xs, the ys, and the lone numbers can be combined to give this expression: "16x+9y+14."