The term change control refers to the process of establishing a defined identification of, a thorough documentation of, submission of a formal approval or rejection of, and Asserting control of all changes that are going to take place to the project baselines. A well defined and adhered to change control process serves multiple purposes within the planning, execution, and finalization of a project. For one, it helps to assure that all changes that do take place have been thoroughly vetted by the project team and project team leader (or for that matter whomever the change control process designates must be involved in the decision making when it comes to making changes). Secondarily, it allows for all changes to be carefully and meticulously kept track of, which allows for fewer surprises along the way, and more accountability at if and when any aspect of the project (whether it be the process or the final deliverables) are called into question. Lastly, a change control can provide financial benefit, in such that it assures that changes that take place are not arbitrary and/or whimsical in nature and rather, have been carefully considered by the team.
This term is defined in the 3rd and the 4th edition of the PMBOK.