The Collaborative International Dictionary of English v.0.48:

Lily \Lil"y\ (l[i^]l"[y^]), n.; pl. Lilies (l[i^]l"[i^]z).
   [AS. lilie, L. lilium, Gr. lei`rion. Cf. Flower-de-luce.]
   1. (Bot.) A plant and flower of the genus Lilium,
      endogenous bulbous plants, having a regular perianth of
      six colored pieces, six stamens, and a superior
      three-celled ovary.
      [1913 Webster]

   Note: There are nearly fifty species, all found in the North
         Temperate zone. Lilium candidum and Lilium
         longiflorum are the common white lilies of gardens;
         Lilium Philadelphicum is the wild red lily of the
         Atlantic States. Lilium Chalcedonicum is supposed to
         be the "lily of the field" in our Lord's parable;
         Lilium auratum is the great gold-banded lily of
         Japan.
         [1913 Webster]

   2. (Bot.) A name given to handsome flowering plants of
      several genera, having some resemblance in color or form
      to a true lily, as Pancratium, Crinum, Amaryllis,
      Nerine, etc.
      [1913 Webster]

   3. That end of a compass needle which should point to the
      north; -- so called as often ornamented with the figure of
      a lily or fleur-de-lis.
      [1913 Webster]

            But sailing further, it veers its lily to the west.
                                                  --Sir T.
                                                  Browne.
      [1913 Webster]

   4. (Auction Bridge) A royal spade; -- usually in pl. See
      Royal spade, below.
      [Webster 1913 Suppl.]

   African lily (Bot.), the blue-flowered Agapanthus
      umbellatus.

   Atamasco lily (Bot.), a plant of the genus Zephyranthes
      (Zephyranthes Atamasco), having a white and pink
      funnelform perianth, with six petal-like divisions
      resembling those of a lily. --Gray.

   Blackberry lily (Bot.), the Pardanthus Chinensis, the
      black seeds of which form a dense mass like a blackberry.


   Bourbon lily (Bot.), Lilium candidum. See Illust.

   Butterfly lily. (Bot.) Same as Mariposa lily, in the
      Vocabulary.

   Lily beetle (Zool.), a European beetle (Crioceris
      merdigera) which feeds upon the white lily.

   Lily daffodil (Bot.), a plant of the genus Narcissus, and
      its flower.

   Lily encrinite (Paleon.), a fossil encrinite, esp.
      Encrinus liliiformis. See Encrinite.

   Lily hyacinth (Bot.), a plant of the genus Hyacinthus.

   Lily iron, a kind of harpoon with a detachable head of
      peculiar shape, used in capturing swordfish.

   Lily of the valley (Bot.), a low perennial herb
      (Convallaria majalis), having a raceme of nodding,
      fragrant, white flowers.

   Lily pad, the large floating leaf of the water lily. [U.
      S.] --Lowell.

   Tiger lily (Bot.), Lilium tigrinum, the sepals of which
      are blotched with black.

   Turk's-cap lily (Bot.) Lilium Martagon, a red lily with
      recurved sepals; also, the similar American lily, Lilium
      superbum.

   Water lily (Bot.), the Nymph[ae]a, a plant with floating
      roundish leaves, and large flowers having many petals,
      usually white, but sometimes pink, red, blue, or yellow.
      [See Illust. of Nymph[ae]a.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

lily pad
    n 1: floating leaves of a water lily