society > travel > travel by water > vessel, ship, or boat > equipment of vessel > masts, rigging, or sails > sail > [noun] > portion which may be reefed (3)
A section of a sail, frequently each of three or four bands or strips, which can be taken in or rolled up to reduce the area exposed to the wind…
the closest reef of a lower fore-and-aft sail, making it nearly triangular, used to steady the ship in stormy weather.