A binomial expression is one that contains two terms, such as , (a+b), (a-b), (3x+y). (5m-n^2) for example. The key is that there must be a + or - between the two terms. https://www.quora.com/What-is-a-binomial-expression A trinomial expression is any polynomial expression that has exactly three terms. https://sciencing.com/solve-trinomials-4500528.html