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Actually, monomial, binomial, and trinomial are based on the number of expressions separated by a + sign or a - sign.

                    4x - monomial ( only one)
                    4/xy - monomial (only one, no +, no -)
                  4xyz - monomial (only one)

4xyz + 3x - binomial ( there are two expressions separated by a + sign: 4xyz and 3x)
4ac - 3/b - binomial (there are two expressions separated by a - sign: 4ac and 3/b)

9ab - 5e - 3 - trinomial (there are three expressions separated: 9ab, 5e and 3
11cd + 4/f - 2y - trinomial (there are three expressions separated: 11cd, 4/f, and 2y