A mechanical organ that turns or is meant to turn on its axis.
In particular, in horology it is the part of a pocket- or wristwatch that oscillates at a steady rhythm.
Some rotors are compensated for thermal dilatation; others can be adjusted by means of weight screws set at the periphery of the rotor.
In the context of helicopters, the rotor is the assembly of blades, hub and pitch control mechanics. Usually you have an horizontal rotor (the big one, responsible for lifting the helicopter and, in most cases, propelling it), and a vertical rotor that keeps the helicopter from spinning on its vertical axis.