So the first step i want to learn before mastering differential equations is 
the classification of differential equations. In general, a differential equation is said to be an equation involving an unknown function (dependent variable ) and its derivatives with respect to one or more independent variables. 

Is $$d^2x/dt^2 + dx/dt = d^2y/dt^2 + y$$ a differential equation?

Does it satisfy the definition given by wikipedia ( A differential equation is a mathematical equation for an unknown function of one or several variables that relates the values of the function itself and its derivatives of various orders ) ?

I don't think so because the definition mentions ONE uknown function, the case i just showed has 2 unknown functions and its derivatives.

Shouldn't the definition be : 

A differential equation is a mathematical equation for ONE OR MORE unknown functions of one or several variables that relates the values of the FUNCTIONS THEMSELVES and its derivatives of various orders with respect to one or more independent variables

?

Clarify my doubt please.