The Collaborative International Dictionary of English v.0.48:

Leaflet \Leaf"let\, n.
   1. A little leaf.
      [1913 Webster]

   2. (Bot.) One of the divisions of a compound leaf; a foliole.
      [1913 Webster]

   3. (Zool.) A leaflike organ or part; as, a leaflet of the
      gills of fishes.
      [1913 Webster]

   4. A printed sheet of paper, of one page, or one sheet folded
      over, containing an advertisement, tract, or other notice,
      and usually distributed for free or included in the
      package with a purchased item.
      [PJC]

WordNet (r) 3.0 (2006):

leaflet
    n 1: a thin triangular flap of a heart valve [syn: cusp,
         leaflet]
    2: part of a compound leaf
    3: a small book usually having a paper cover [syn: booklet,
       brochure, folder, leaflet, pamphlet]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "leaflet":
   advert, advertisement, bill, blade, booklet, bract, bracteole,
   bractlet, brochure, chapbook, circular, comic book, cotyledon,
   flag, flier, floral leaf, flyer, folder, foliole, frond, glume,
   handbill, involucre, involucrum, lamina, leaf, lemma, ligule,
   needle, pamphlet, petal, pile, pine needle, seed leaf, sepal,
   spathe, spear, spire, stipula, stipule, tract