mainsail

noun

1. The largest (or only) sail on a sailing vessel.

   “The mainsail was "scandalised" - a nautical mode of describing a sail reefed at both ends[.]”

   “Now stow the mainsail as follows, preferably with the sail still lying in the starboard cockpit: beginning at the head, fold the upper section so that the upper two battens are aligned together; then tightly roll the sail from the head down to the foot. Reinsert the sail into its bag.”