A mathematical phrase used in Algebra. Like terms refers to various monomials, binomials, and polynomials.

2, 7, 3.14, and 1/2 are like terms. Numbers are like terms.

5x, -90x, and .75x are like terms. Variables that use the same letters are like terms.

In sum (pun intended) like terms are terms that have the same variable coefficients.

Why would use use like terms? Because when simplifying an equation, you can only use like terms. If you do otherwise, you'll get the ever annoying wrong answer.

/me is amazed someone didn't node this earlier.