Flight Controls Perspective
Working as an engineer in both flight controls and propulsion I have used this discriminator:
If it's primary purpose is solely for propulsion and it dose not have a means for high frequency blade pitch control it is a propeller.
If it's primary purpose is either solely flight control or equally flight control and propulsion, plus it is equipped with a means for high frequency blade pitch control it is a rotor.
Note that on small quad copters that use variable rpm they are called propellers. But on quad copters with collective pitch control they are called rotors.
CTR