The minimum en-route altitude (MEA) is the altitude for an en-route segment that provides adequate reception of relevant navigation facilities and ATS communications, complies with the airspace structure and provides the required obstacle clearance. (ICAO Doc 8168 - PANS-OPS)
Put simply, the MEA is the lowest altitude to be flown in an airspace structure which assures:
reception of navigation aids necessary to navigate accurately along the required route,
two-way communication with air traffic control,
safe clearance from obstacles within the sector, and
conformity with any air traffic control procedures applicable within the sector.