The Collaborative International Dictionary of English v.0.48:

del \del\ n. (Math.) a differential operator which, operating on a function of several variables, gives the sum of the partial derivatives of the function with respect to the three orthogonal spatial coordinates; -- also called the gradient or grad. It is represented by an inverted Greek capital delta ([nabla]), and is thus because of its shape also called nabla, meaning harp in Hebrew. [PJC]