main·sail 
(mānsəl, -sāl′)
(mānsəl, -sāl′)n.
1. The principal sail of a vessel.
2. A quadrilateral or triangular sail set from the after part of the mainmast on a fore-and-aft-rigged vessel.
3. A square sail set from the main yard on a square-rigged vessel. See Usage Note at boatswain.