The Collaborative International Dictionary of English v.0.48:
mainsheet \main"sheet`\, n. (Naut.) One of the ropes by which the mainsail is hauled aft and trimmed. [1913 Webster]WordNet (r) 3.0 (2006):
mainsheet n 1: (nautical) a line (rope or chain) that regulates the angle at which a sail is set in relation to the wind [syn: sheet, tack, mainsheet, weather sheet, shroud]