mainsail(n = noun.artifact) - the lowermost sail on the mainmast;
is a kind of canvas, canvass, sail, sheet
has particulars: main course

   The principal sail in a ship or other vessel. [1913 Webster]
" 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]
"[They] hoised up the mainsail to the wind." [1913 Webster]

mainsail, n. Naut.
1 (in a square-rigged vessel) the lowest sail on the mainmast.
2 (in a fore-and-aft rigged vessel) a sail set on the after part of the mainmast.