f is by default interpreted as Del[f].
Used in vector analysis to denote gradient operator and its generalizations.
Used in numerical analysis to denote backward difference operator.
Also called nabla.
Not the same as \[EmptyDownTriangle].
See The Mathematica Book: Section 3.10.3 and Section 3.10.4.
See also: \[CapitalDelta] , \[PartialD] , \[Square] .