A monomial is any product of numbers and variables, like 17, or 3xy, or –4x2, or
The only rules are that the variables should be raised to only positive integer powers (no square roots or 1/x's allowed), and no plus or minus signs.
A binomial is the sum of two monomials, for example x + 3, or 55x2 – 33y2, or
A trinomial is the sum of three monomials.
A polynomial is the sum of n monomials, for some whole number n. So monomials, binomials and trinomials are all special cases of polynomials. A polynomial can have as many terms as you want.