What are the requirements for terms to be like terms?

They both have to end with a letter like 5x and 9x are like terms

For terms to be like terms, they must have the same variables raised to the same power.

For example, 4x and 3x^2 are not like terms since although they both have x, the x's are raised to different powers. However, 4x^2 and 3x^2 are like terms since they both have only x and for both the x is squared.