Isolated Danger Mark (or Buoy) (G.B.)
Alternative term: Obstruction Mark (or Buoy) (U.S.A.)
A mark (or buoy) used alone to indicate a dangerous reef or shoal. The mark (or buoy) may be passed on either hand.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM