CMMI is an acronym (it is not a four-letter word); it stands for Capability Maturity Model Integration. Some people would say that the CMMI is a model with multiple representation while others would describe it as a set of models. But most will agree that the CMMI is a merger of process improvement models for systems engineering, software engineering, integrated product development, and software acquisition.
Some of the goals of the CMMI are to provide a common vocabulary across the set of models and to provide clarification on how these areas interrelate. The integrated model has both a continuous and staged architecture.
[1] CMMI and CMM are registered in the U.S. Patent and Trademark office by Carnegie Mellon University.