A higher partial derivative involving more than one independent variable; for example $\frac{{\partial }^{3}f}{{\partial }^{2}x\partial y}.$ If the mixed partial derivatives $\frac{{\partial }^{2}f}{\partial x\partial y}$ and $\frac{{\partial }^{2}f}{\partial y\partial x}$ both exist and are continuous functions, then they are equal; that is, the order of partial differentiation does not matter.