The CMMI Glossary defines the basic terms used in the CMMI, especially those terms that have a specific mean in the context of the CMMI. Many of the definitions are for multiple-word terms. If the Glossary does not contain a definition, then the common usage or other dictionary definition is applicable.
The SEI's Introduction to CMMI class spends some time stressing the importance of the Glossary and the need to consult it whenever a question arises.
Despite all this, the CMMI Glossary, next to the Generic Goals and Generic Practices section, is probably the most overlooked area of the model. People tend to remain focussed on reading the Process Areas and rather than consult the Glossary for specific definitions and also tend to apply their own definitions and interpretations, which can lead to improper implementations. Plus it doesn't help that the Glossary is at the back of the book.
For example, the "CMMI lawyers" can start to argue that documented policies and procedures are not required because there are no explicit words in the CMMI stating documented policies or procedures. What the "lawyers" are missing is the special CMMI meaning of the phrase "establish and maintain." This phrase is emphasized in the Intro to CMMI class and is defined in the Glossary as formulate, document, and use. So in GP 2.1 where it states "establish and maintain an organizational policy for ..." that means to think about the wording of the policy, talk about it and achieve consensus, write it down or document it, and then use it.
So, the lesson here is whenever you have a question about the CMMI, the first place you should go is the Glossary and see if the term or phrase in question has been defined in the context of the CMMI. There is a lot of good information contained in the Glossary. Granted, it is not complete, so you may expect to find a definition here that is missing. If you disagree with a Glossary definition and/or wish to include a definition, you can always write a Change Request to the Glossary and submit it to the SEI for consideration.