Coaxial rotors are a pair of helicopter rotors mounted one above the other on concentric shafts, with the same axis of rotation, but that turn in opposite directions (contra-rotation). This configuration is a feature of helicopters produced by the Russian Kamov helicopter design bureau.
Read more about Coaxial Rotors: History, Coaxial Models, Hazards of Helicopter Flight, List of Co-axial Rotor Helicopters