The Collaborative International Dictionary of English v.0.48:

Calyx \Ca"lyx\ (k[=a]"l[i^]ks; 277), n.; pl. E. Calyxes
   (k[=a]"l[i^]ks*[e^]z), L. Calyces (k[a^]l"[i^]*s[=e]z). [L.
   calyx, -ycis, fr. Gr. ka`lyx husk, shell, calyx, from the
   root of kaly`ptein to cover, conceal. Cf. Chalice Helmet.]
   1. (Bot.) The covering of a flower. See Flower.
      [1913 Webster]

   Note: The calyx is usually green and foliaceous, but becomes
         delicate and petaloid in such flowers as the anemone
         and the four-o'clock. Each leaf of the calyx is called
         a sepal.
         [1913 Webster]

   2. (Anat.) A cuplike division of the pelvis of the kidney,
      which surrounds one or more of the renal papill[ae].
      [1913 Webster]

WordNet (r) 3.0 (2006):

calyx
    n 1: (botany) the whorl of sepals of a flower collectively
         forming the outer floral envelope or layer of the perianth
         enclosing and supporting the developing bud; usually green