'MAINSAIL' definitions:

Definition of 'mainsail'

noun

The lowermost sail on the mainmast

From: GCIDE

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]

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).