any operator involving differentiation, such as the mathematical operator del ∇, used in vector analysis, where ∇ = i∂/∂ x + j ∂/∂ y + k ∂/∂ z, i, j, and k being unit vectors and ∂/∂x, ∂/∂y, and ∂/∂z the partial derivatives of a function in x, y, and z