What is OOP, really? Why so many different definitions?

Which OOP? Alan Kay meant this:

OOP to me means only messaging, local retention and protection and hiding of state-process, and extreme late-binding of all things. It can be done in Smalltalk and in LISP. There are possibly other systems in which this is possible, but I'm not aware of them.

But there is also various other OOP around. And those really about completly different things.