mainsail
 oxford
 views  updated Jun 27 2018main·sail  / ˈmānsəl; -ˌsāl/ •
n. the principal sail of a ship, esp. the lowest sail on the mainmast in a square-rigged vessel. ∎  the sail set on the after side of the mainmast in a fore-and-aft-rigged vessel.
 The Oxford Pocket Dictionary of Current English