Easton's Bible Dictionary (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). Noah Webster's Dictionary (n.) The principal sail in a ship or other vessel.

Multi-Version Concordance Mainsail (1 Occurrence) Acts 27:40 And when they had taken up the anchors, they committed themselves unto the sea, and loosed the rudder bands, and hoised up the mainsail to the wind, and made toward shore. (KJV WBS YLT)