The Collaborative International Dictionary of English v.0.48:

Bract \Bract\, n. [See Bractea.] (Bot.)
   (a) A leaf, usually smaller than the true leaves of a plant,
       from the axil of which a flower stalk arises.
   (b) Any modified leaf, or scale, on a flower stalk or at the
       base of a flower.
       [1913 Webster]

   Note: Bracts are often inconspicuous, but sometimes large and
         showy, or highly colored, as in many cactaceous plants.
         The spathes of aroid plants are conspicuous forms of
         bracts.
         [1913 Webster]

WordNet (r) 3.0 (2006):

bract
    n 1: a modified leaf or leaflike part just below and protecting
         an inflorescence

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "bract":
   blade, bracteole, bractlet, cotyledon, flag, floral leaf, foliole,
   frond, glume, involucre, involucrum, lamina, leaf, leaflet, lemma,
   ligule, needle, petal, pile, pine needle, seed leaf, sepal, spathe,
   spear, spire, stipula, stipule