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]