A polynomial is an expression which is a monomial, or the sum of two or more monomials.

The expression 2x + 3x contains like terms which add to 5x. The monomial 5x can be expressed as the binomial 2x + 3x.

The expression 2x + 3y contains unlike terms which cannot be added. The binomial 2x + 3y can be expressed as the monomial (2x + 3y).

Usually the simplified form is used when classifying an expression, unless the context of a situation requires a non-simplified form to be used.