total differential
The total differential of a function f = f(x1, x2, · · ·, xn) of n variables is defined by the equation
whether or not x1, x2, · · ·, and xn are independent of each other. 
, 
, · · ·, and 
denote the partial derivatives of f with respect to x1, x2, · · ·, and xn respectively. The total differential df represents the change in f associated with simultaneous infinitesimal changes in x1, x2, · · ·, and xn given by dx1, dx2, · · ·, dxn, respectively, to the order of dx1, dx2, · · ·, dxn.