And also:The zero of a function is the x-value (valor de x) that makes the function equal to 0.
Generally, they use the word function, because (they say, more technically) zero is of a function and one of the roots of an equation.
So, in general terms I understand that when we are solving for one variable ( x ) we talk about functions, when we are talking about more than one variable x and y, we are talking about equations. In general terms I mean; I don´t want to dig deeper and make it hard, it´s not Maths class after all.
Anyway, feel free to correct me if I am mistaken.