A trinomial is an expression with exactly three terms that can't be combined an example would be this
x^2 - 3x + 1
None of those terms can be combined
x^2 + 3 - 4 has three terms but it isn't considered a trinomial because you can simplify it to x^2 - 1 which is known as a binomial because it has 2 terms in it's simplified form.
