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

The Free On-line Dictionary of Computing (19 January 2023):

MAINSAIL

   MAchine INdependent SAIL.