Source : Webster's Revised Unabridged Dictionary (1913)

Mainsheet \Main"sheet`\, n. (Naut.)
   One of the ropes by which the mainsail is hauled aft and
   trimmed.

Source : WordNet®

mainsheet
     n : (nautical) a line (rope or chain) that regulates the angle
         at which a sail is set in relation to the wind [syn: {sheet},
          {tack}, {weather sheet}, {shroud}]