Why is it impossible to have a linear trinomial with one variable?

The linear in "linear trinomial" states that the degree of any monomial (single variable) must be equal to 1. A trinomial implies that there are three variables. An example of a trinomial would be 2x - y + 3z, where there are three variables each to the first degree. Therefore, a linear trinomial could not have only one variable since three variables are required.