The term Change of Status (COS) is the name given to the process through which a person can change from one temporary status (nonimmigrant) to another temporary status without having to leave the United States. Typical examples of this process are changing from a student visa status to H-1B. Usually, a successful COS is evidenced by the USCIS issuing you a new I-94.
Changing status from a temporary visa to a green card within the United States is also a change of status, but it is technically referred to as Adjustment of Status (AOS).