As one major U.S. manufacturer (starting with a B) puts it -
High Speed Cruise is cruise at either the limiting speed for the aircraft (Vmo/Mmo) OR the maximum speed attainable at Maximum Cruise Thrust (MCR) if less than Vmo/Mmo. Performance tables applicable to High Speed Cruise conform to this philosophy.
Long Rance Cruise is as Clandestino described it.
Regards,
Old Smokey