An expression consists of at least two terms and an operation, such as multiply, divide, add or subtract. It does not have an equals sign. When you add an equals sign to an expression it becomes an equation, and there is obviously a term on both sides.

You need to remember that an algebraic expression combines variables (usually specific letters), constants(real numbers) andalgebraic operations (addition/subtraction, multiplication/division, exponentiation).

A polynomial represents an algebraic expression such that:

2x^2 + 3yz + 5^(1/2)