In Algebra, an expression is a set of one or more terms. For instance, in the equation

7x - 2y + 3z = 3x + 7y - 2z

7x - 2y + 3z is an expression, as is 3x + 7y - 2z.

However, 7x - 2y + 3z = 3x + 7y - 2z is not in and of itself an expression.

These expressions are given diffrent names depending on the number of terms involved.

An expression with one term, like 7x alone, is called a monomial.
An expression with two terms, like 3x + 4y, is called a binomial.
An expression with three terms, like 5x + 8z - 3xz, is called a trinomial.