The Collaborative International Dictionary of English v.0.48:

Christmas \Christ"mas\, n. [Christ + mass.]
   An annual church festival (December 25) and in some States a
   legal holiday, in memory of the birth of Christ, often
   celebrated by a particular church service, and also by
   special gifts, greetings, and hospitality.
   [1913 Webster]

   Christmas box.
   (a) A box in which presents are deposited at Christmas.
   (b) A present or small gratuity given to young people and
       servants at Christmas; a Christmas gift.

   Christmas carol, a carol sung at, or suitable for,
      Christmas.

   Christmas day. Same as Christmas.

   Christmas eve, the evening before Christmas.

   Christmas fern (Bot.), an evergreen North American fern
      (Aspidium acrostichoides), which is much used for
      decoration in winter.

   Christmas flower, Christmas rose, the black hellebore, a
      poisonous plant of the buttercup family, which in Southern
      Europe often produces beautiful roselike flowers
      midwinter.

   Christmas tree, a small evergreen tree, set up indoors, to
      be decorated with bonbons, presents, etc., and illuminated
      on Christmas eve.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fern \Fern\ (f[~e]rn), n. [AS. fearn; akin to D. varen, G. farn,
   farnkraut; cf. Skr. par[.n]a wing, feather, leaf, sort of
   plant, or Lith. papartis fern.] (Bot.)
   An order of cryptogamous plants, the Filices, which have
   their fructification on the back of the fronds or leaves.
   They are usually found in humid soil, sometimes grow
   epiphytically on trees, and in tropical climates often attain
   a gigantic size.
   [1913 Webster]

   Note: The plants are asexual, and bear clustered sporangia,
         containing minute spores, which germinate and form
         prothalli, on which are borne the true organs of
         reproduction. The brake or bracken, the maidenhair, and
         the polypody are all well known ferns.
         [1913 Webster]

   Christmas fern. See under Christmas.

   Climbing fern (Bot.), a delicate North American fern
      (Lygodium palmatum), which climbs several feet high over
      bushes, etc., and is much sought for purposes of
      decoration.

   Fern owl. (Zool.)
   (a) The European goatsucker.
   (b) The short-eared owl. [Prov. Eng.] -- Fern shaw, a fern
      thicket. [Eng.] --R. Browning.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Christmas fern
    n 1: North American evergreen fern having pinnate leaves and
         dense clusters of lance-shaped fronds [syn: Christmas
         fern, canker brake, dagger fern, evergreen wood
         fern, Polystichum acrostichoides]