The Collaborative International Dictionary of English v.0.48:

Petal \Pet"al\ (p[e^]t"al; 277), n. [Gr. pe`talon a leaf, a leaf
   or plate of metal, fr. pe`talos outspread, broad, flat: cf.
   F. p['e]tale. See Fathom.]
   1. (Bot.) One of the leaves of the corolla, or the colored
      leaves of a flower. See Corolla, and Illust. of
      Flower.
      [1913 Webster]

   2. (Zool.) One of the expanded ambulacra which form a rosette
      on the black of certain Echini.
      [1913 Webster]

WordNet (r) 3.0 (2006):

petal
    n 1: part of the perianth that is usually brightly colored [syn:
         petal, flower petal]