“Rotating” to get AVL Tree

Question

Why is the process of balancing to get an AVL tree called rotation? (While you are at it, what's single & double rotation?)

Every textbook of mine blatantly uses that word without any explanation.

Answer 1

This is a operation of changing root in sub-tree of your tree, this is very simple operation and this can not be explained (because is simple).

Answer 2

See Wikipedia.