binomial
A binomial is a polynomial with exactly two terms — two building blocks joined by a single plus or minus sign. Things like x + 3, 2a - 5b, or x^2 - 9 are all binomials. The prefix bi- means two, just as in bicycle (two wheels).
Binomials show up constantly because so many useful patterns involve two pieces: the square of a binomial, the difference of two squares, and the whole FOIL shortcut for multiplying two of them. Learning to handle binomials smoothly is one of the first big payoffs in algebra.
Be careful that the two terms must be genuinely unlike — they should not be combinable into one. For example 3x + 5x is not really a binomial, because those are like terms that collapse to 8x, a single term. A true binomial like 3x + 5 stays at two terms no matter how you simplify.
x^2 - 9 is a binomial, and it happens to be a difference of two squares: x^2 - 9 = (x + 3)(x - 3).
Two terms — many of algebra's neatest patterns live here.