[1] : The Collaborative International Dictionary of English v.0.48

mainsail \main"sail`\ m=an"s=al`, n. naut.
   the principal sail in a ship or other vessel.
   1913 webster

         they hoised up the mainsail to the wind. -- acts
                                                  xxvii. 40.
   1913 webster

   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.
         1913 webster

[2] : WordNet (r) 2.0

mainsail
     n : the lowermost sail on the mainmast

[3] : Virtual Entity of Relevant Acronyms (Version 1.9, June 2002)

mainsail
     machine independent sail sail
     
     

[4] : The Free On-line Dictionary of Computing (27 SEP 03)

mainsail
     
        machine independent sail.  from xidak, palo alto ca, 415
        855-9271.
     
     

[5] : Easton's 1897 Bible Dictionary

main-sail
   gr. artemon, answering to the modern "mizzen-sail," as some
   suppose. others understand the "jib," near the prow, or the
   "fore-sail," as likely to be most useful in bringing a ship's
   head to the wind in the circumstances described acts 27:40.
   

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