The Collaborative International Dictionary of English v.0.48:

Reef \Reef\ (r[=e]f), n. [Akin to D. rif, G. riff, Icel. rif,
   Dan. rev; cf. Icel. rifa rift, rent, fissure, rifa to rive,
   bear. Cf. Rift, Rive.]
   1. A chain or range of rocks lying at or near the surface of
      the water. See Coral reefs, under Coral.
      [1913 Webster]

   2. (Mining.) A large vein of auriferous quartz; -- so called
      in Australia. Hence, any body of rock yielding valuable
      ore.
      [1913 Webster]

   Reef builder (Zool.), any stony coral which contributes
      material to the formation of coral reefs.

   Reef heron (Zool.), any heron of the genus Demigretta;
      as, the blue reef heron (Demigretta jugularis) of
      Australia.
      [1913 Webster]