An Etymological Dictionary of Astronomy and Astrophysics

In → vector calculus, a vector → partial derivative represented by the symbol → nabla and defined in three dimensions to be:
∇ = (∂/∂x)i + (∂/∂y)j

• (∂/∂z)k.
  It appears in the operations → gradient: ∇f, → divergence: ∇ . E, → curl: ∇ x E, and → Laplacian:
  ∇ . ∇f = ∇²f.

See also: From Gk. alphabet letter delta.