Wordnet 3.0
NOUN (1)
1. 
 rotor consisting of large rotating airfoils on a single-rotor helicopter that produce the lift to support the helicopter in the air; 
The Collaborative International Dictionary of English v.0.48:
main rotor \main rotor\ n. (Aviation)
   The assembly of large rotating airfoils (blades) on a
   helicopter that produce the lift to support the helicopter in
   the air.
   [WordNet 1.5]
WordNet (r) 3.0 (2006):
main rotor
    n 1: rotor consisting of large rotating airfoils on a single-
         rotor helicopter that produce the lift to support the
         helicopter in the air