Question

what does "del" mean as nabla

Answer 1

del is the vector differential operator, represented by an upside down triangle.

Answer 2

http://en.wikipedia.org/wiki/Del

Answer 3

Del is used when taking the magnitude of the partial derivatives kind of like when you are doing arc length with a vector valued function. There are times in which del is not explicitly defined but that comes more often in advanced vector calculus and tensor analysis.