The Art of the Metaobject Protocol, a 1991 book by Gregor Kiczales, Jim des Rivieres, and Daniel G. Bobrow (all three working for Xerox PARC). In his 1997 talk at OOPSLA, Alan Kay called it "the best book anybody's written in ten years", and contended that it contained "some of the most profound insights, and the most practical insights about OOP", but was dismayed that it was written in a highly Lisp-centric and CLOS-specific fashion, calling it "a hard book for most people to read; if you don't know the Lisp culture, it's very hard to read". wikipedia
A meta object protocol is an API to an object system. perldoc
To be more specific, it abstracts the components of an object system (classes, object, methods, object attributes, etc.). These abstractions can then be used to inspect and manipulate the object system which they describe.
It can be said that there are two MOPs for any object system; the implicit MOP and the explicit MOP. The implicit MOP handles things like method dispatch or inheritance, which happen automatically as part of how the object system works. The explicit MOP typically handles the introspection/reflection features of the object system.
All object systems have implicit MOPs. Without one, they would not work. Explicit MOPs are much less common, and depending on the language can vary from restrictive (Reflection in Java or C#) to wide open (CLOS is a perfect example).