The Collaborative International Dictionary of English v.0.48:

Parasite \Par"a*site\ (p[a^]r"[.a]*s[imac]t), n. [F., fr. L.
   parasitus, Gr. para`sitos, lit., eating beside, or at the
   table of, another; para` beside + sitei^n to feed, from sitos
   wheat, grain, food.]
   [1913 Webster]
   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.
      [1913 Webster]

            Thou, with trembling fear,
            Or like a fawning parasite, obey'st.  --Milton.
      [1913 Webster]

            Parasites were called such smell-feasts as would
            seek to be free guests at rich men's tables.
                                                  --Udall.
      [1913 Webster]

   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.
          [1913 Webster]

   3. (Zool.)
      (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.
          [1913 Webster] Parasitic

WordNet (r) 3.0 (2006):

parasite
    n 1: an animal or plant that lives in or on a host (another
         animal or plant); it 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, parasite,
       sponge, sponger]

Moby Thesaurus II by Grady Ward, 1.0:

149 Moby Thesaurus words for "parasite":
   adherent, algae, appendage, attendance, attendant, autophyte,
   barnacle, bean, beat, bedbug, beggar, bloodsucker,
   body of retainers, bracken, brown algae, buff, bummer, cadger,
   cat flea, cavaliere servente, chigoe, climber, cockroach, cohort,
   conferva, confervoid, cortege, coupon clippers, court, courtier,
   crab, creeper, dangler, deadbeat, dependent, diatom, disciple,
   dog flea, drone, entourage, fan, fern, flea, flunky, follower,
   following, freeloader, fruits and vegetables, fucus, fungus,
   grapevine, grayback, green algae, gulfweed, hanger-on, henchman,
   herb, heterophyte, homme de cour, hyena, idle rich, idler, ivy,
   jackal, jigger, kelp, laze, leech, legume, leisure class, lentil,
   liana, lichen, liverwort, lounge lizard, louse, lumpen proletariat,
   mendicant, mite, mold, moocher, mosquito, moss, mushroom, nit,
   nonworker, panhandler, parasitic plant, parasitize, partisan, pea,
   perthophyte, phytoplankton, planktonic algae, plant families,
   public, puffball, pulse, pursuer, pursuivant, red algae, red bug,
   rentiers, retinue, roach, rockweed, rout, rust, sand flea,
   saprophyte, sargasso, sargassum, satellite, scrounge, scrounger,
   sea lentil, sea moss, sea wrack, seaweed, sectary, shadow,
   smell-feast, smut, spiv, sponge, sponger, stooge, successor,
   succulent, sucker, suite, supporter, tagtail, tail,
   the unemployable, the unemployed, tick, toadstool, train,
   trainbearer, vermin, vetch, vine, votary, ward heeler, weevil,
   wood tick, wort, wrack