Source : Webster's Revised Unabridged Dictionary (1913)

Parasite \Par"a*site\, n. [F., fr. L. parasitus, Gr. ?, lit.,
   eating beside, or at the table of, another; ? beside + ? to
   feed, from ? wheat, grain, food.]
   1. One who frequents the tables of the rich, or who lives at
      another's expense, and earns his welcome by flattery; a
      hanger-on; a toady; a sycophant.

            Thou, with trembling fear, Or like a fawning
            parasite, obey'st.                    --Milton.

            Parasites were called such smell-feasts as would
            seek to be free guests at rich men's tables.
                                                  --Udall.

   2. (Bot.)
      (a) A plant obtaining nourishment immediately from other
          plants to which it attaches itself, and whose juices
          it absorbs; -- sometimes, but erroneously, called
          epiphyte.
      (b) A plant living on or within an animal, and supported
          at its expense, as many species of fungi of the genus
          {Torrubia}.

   3. (Zo["o]l.)
      (a) An animal which lives during the whole or part of its
          existence on or in the body of some other animal,
          feeding upon its food, blood, or tissues, as lice,
          tapeworms, etc.
      (b) An animal which steals the food of another, as the
          parasitic jager.
      (c) An animal which habitually uses the nest of another,
          as the cowbird and the European cuckoo.

Source : WordNet®

parasite
     n 1: an animal or plant that lives in or on a host (another
          animal or plant); the parasite obtains nourishment from
          the host without benefiting or killing the host [ant: {host}]
     2: a follower who hangs around a host (without benefit to the
        host) in hope of gain or advantage [syn: {leech}, {sponge},
         {sponger}]