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]