Plain English Meaning
The lowest altitude a controller is allowed to send you to when they are steering you with radar vectors. It guarantees you stay safely above terrain and obstacles in that piece of airspace.
Full Definition
The lowest altitude, expressed in feet above mean sea level, at which an air traffic controller may issue radar vectors to an aircraft within a specific area of the controller's airspace. The MVA provides clearance above terrain and obstacles and meets communication and radar coverage requirements. MVAs are established for each radar sector and are depicted on the controller's radar display, but are not published on to pilots.