Source : Webster's Revised Unabridged Dictionary (1913)

Mainsail \Main"sail`\, n. (Naut.)
   The principal sail in a ship or other vessel.

         [They] hoised up the mainsail to the wind. -- Acts
                                                  xxvii. 40.

   Note: The mainsail of a ship is extended upon a yard attached
         to the mainmast, and that of a sloop or schooner upon
         the boom.

Source : WordNet®

mainsail
     n : the lowermost sail on the mainmast

Source : Free On-Line Dictionary of Computing

MAINSAIL
     
        MAchine INdependent SAIL.  From XIDAK, Palo Alto CA, (415)
        855-9271.