The two terms seem interchangeable and I can't find a strict definition for both, so what's the difference between them if there is any?
An impeller is the type of rotor found in a centrifugal pump, in fact another name for "centrifugal pumps" is "impeller pumps". Many different types of pump have rotors.
The pump "rotor" is simply a term the rotating components of the pump. Gear pumps, sliding vane pumps, swash-plate pumps, and circumferential piston pumps all have rotors, but none of them have impellers