total derivative

The rate of change of a function of two or more variables with reference to a parameter on which these variables are dependent. If z = f(x, y, t), x = φ(t), and y = θ(t), the total derivative of z with reference to t, written dz/dt, is given by

where and are the partial derivatives of f(x, y) with respect to x and y, respectively; and are the total derivatives of φ(t) and θ(t), respectively, with respect to t. For a function of one variable, the partial derivative equals the total derivative. The individual derivative is the total derivative when the parameter t represents time.