The Collaborative International Dictionary of English v.0.48:

Fleur-de-lis \Fleur`-de-lis`\, n.; pl. Fleurs-de-lis. [F.,
   flower of the lily. Cf. Flower-de-luce, Lily.]
   1. (Bot.) The iris. See Flower-de-luce.
      [1913 Webster]

   2. A conventional flower suggested by the iris, and having a
      form which fits it for the terminal decoration of a
      scepter, the ornaments of a crown, etc. It is also a
      heraldic bearing, and is identified with the royal arms
      and adornments of France.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fleur-de-lis
    n 1: plants with sword-shaped leaves and erect stalks bearing
         bright-colored flowers composed of three petals and three
         drooping sepals [syn: iris, flag, fleur-de-lis,
         sword lily]
    2: (heraldry) charge consisting of a conventionalized
       representation of an iris [syn: fleur-de-lis, fleur-de-
       lys]