The word “power” is in the physics sense, referring to the amount of power expended by a cartridge and delivered by the bullet. What counts as high powered is somewhat subjective, but most big game hunting cartridges are high powered, and most handgun and modern sporting rifle cartridges are not. Though you will often hear the term “high powered rifle”, it is characteristic of the cartridge more than the gun that fires it. Most importantly, this term does not mean “very shooty”. AR-15 rifles are not high powered.
When CNN called the KelTec CP33 with a brace and a supressor a “high powered rifle”, they demonstrated their ignorance. The KelTec CP33 fires .22 Long Rifle ammunition, which is among the lowest power cartridges made.