NABLA: What is it?

I’m just curious about what the Nabla value means in most procedural textures. I tried tweaking the value but it doesn’t seem to change anything. But according to BlenderWiki, Nabla means:

  • “Almost all procedural textures in Blender use derivatives for calculating normals for texture mapping (with as exception Blend and Magic). This is important for Normal and Displacment Maps. The strength of the effect is controlled with the Nabla Number Button.”
Please help me interpret that? :D

There is a bit more on this. Also math and some answers to why.

well one meaning is as follow

you have nabla = G direction of gravity for normal

zenith is vertical upward like - G


Deeply confusing but Thanks.