The Collaborative International Dictionary of English v.0.48:

Mainsail \Main"sail`\ (m[=a]n"s[=a]l`), 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]

WordNet (r) 3.0 (2006):

mainsail
    n 1: the lowermost sail on the mainmast

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

MAINSAIL
       MAchine INdependent SAIL (SAIL)


The Free On-line Dictionary of Computing (30 December 2018):

MAINSAIL

   MAchine INdependent SAIL.


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).