"Rotating" to get AVL Tree

Why is the process of balancing to get an A开发者_Go百科VL tree called rotation? (While you are at it, what's single & double rotation?)

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

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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).

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See Wikipedia.