Generally, any operator involving derivatives or partial derivatives. In particular, the operator del $\nabla =i\frac{\partial }{\partial x}+j\frac{\partial }{\partial y}+k\frac{\partial }{\partial z}$, where i, j, k are unit vectors in directions Ox, Oy, Oz, and $\frac{\partial }{\partial x},\frac{\partial }{\partial y},\frac{\partial }{\partial z}$ are the partial derivatives of the function with respect to x, y, z. Note that the operator del does not behave as vectors do (see appendix 16). See also curl, divergence, gradient, Laplacian.