Non-scheduled air service
a commercial air transport service performed as other than a scheduled air service.
ICAO (2004), doc. 9626, Manual on the Regulation of International Air Transport
Though the terms non-scheduled and charter (i.e. a contractual arrangement between an air carrier and an entity hiring or leasing its aircraft) have come to be used interchangeably, it should be noted that not all commercial non-scheduled operations are charter flights. (same source)