The Collaborative International Dictionary of English v.0.48:

Sedge \Sedge\, n. [OE. segge, AS. secg; akin to LG. segge; --
   probably named from its bladelike appearance, and akin to L.
   secare to cut, E. saw a cutting instrument; cf. Ir. seisg, W.
   hesg. Cf. Hassock, Saw the instrument.]
   1. (Bot.) Any plant of the genus Carex, perennial,
      endogenous, innutritious herbs, often growing in dense
      tufts in marshy places. They have triangular jointless
      stems, a spiked inflorescence, and long grasslike leaves
      which are usually rough on the margins and midrib. There
      are several hundred species.
      [1913 Webster]

   Note: The name is sometimes given to any other plant of the
         order Cyperaceae, which includes Carex, Cyperus,
         Scirpus, and many other genera of rushlike plants.
         [1913 Webster]

   2. (Zool.) A flock of herons.
      [1913 Webster]

   Sedge hen (Zool.), the clapper rail. See under 5th Rail.


   Sedge warbler (Zool.), a small European singing bird
      (Acrocephalus phragmitis). It often builds its nest
      among reeds; -- called also sedge bird, sedge wren,
      night warbler, and Scotch nightingale.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sedge
    n 1: grasslike or rushlike plant growing in wet places having
         solid stems, narrow grasslike leaves and spikelets of
         inconspicuous flowers