The ordinary derivative of a function of two or more variables with respect to one of the variables, the others being considered constants. If the variables are x and y, the partial derivatives of F(x, y) are written ∂f/∂x and ∂f/∂y, or D_xf and D_yf, or f_x and f_y. The partial derivative of a variable with respect to time is known as the local derivative.