The Collaborative International Dictionary of English v.0.48:

Calamus \Cal"a*mus\, n.; pl. Calami. [L., a reed. See Halm.]
   1. (Bot.) The indian cane, a plant of the Palm family. It
      furnishes the common rattan. See Rattan, and Dragon's
      blood.
      [1913 Webster]

   2. (Bot.) A species of Acorus (Acorus calamus), commonly
      called calamus, or sweet flag. The root has a pungent,
      aromatic taste, and is used in medicine as a stomachic;
      the leaves have an aromatic odor, and were formerly used
      instead of rushes to strew on floors.
      [1913 Webster]

   3. (Zool.) The horny basal portion of a feather; the barrel
      or quill.
      [1913 Webster]

WordNet (r) 3.0 (2006):

calamus
    n 1: any tropical Asian palm of the genus Calamus; light tough
         stems are a source of rattan canes
    2: the aromatic root of the sweet flag used medicinally
    3: perennial marsh plant having swordlike leaves and aromatic
       roots [syn: sweet flag, calamus, sweet calamus, myrtle
       flag, flagroot, Acorus calamus]
    4: a genus of Sparidae [syn: Calamus, genus Calamus]
    5: the hollow spine of a feather [syn: quill, calamus,
       shaft]