The Collaborative International Dictionary of English v.0.48:

Flagellum \Fla*gel"lum\, n.; pl. E. Flagellums, L. Flagella.
   [L., a whip. See Flagellate, v. t.]
   1. (Bot.) A young, flexible shoot of a plant; esp., the long
      trailing branch of a vine, or a slender branch in certain
      mosses.
      [1913 Webster]

   2. (Zool.)
      (a) A long, whiplike cilium. See Flagellata.
      (b) An appendage of the reproductive apparatus of the
          snail.
      (c) A lashlike appendage of a crustacean, esp. the
          terminal ortion of the antenn[ae] and the epipodite of
          the maxilipeds. See Maxilliped.
          [1913 Webster]

WordNet (r) 3.0 (2006):

flagellum
    n 1: a whip used to inflict punishment (often used for pedantic
         humor) [syn: scourge, flagellum]
    2: a lash-like appendage used for locomotion (e.g., in sperm
       cells and some bacteria and protozoa)

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "flagellum":
   Golgi apparatus, animal fiber, artificial fiber, aster, basal body,
   belt, bine, blacksnake, bough, branch, branchedness, branchiness,
   bullwhack, bullwhip, burgeon, capillament, cat, cell membrane,
   central apparatus, central body, centriole, centroplasm,
   centrosome, centrosphere, chloroplast, chromoplast, cilia, cilium,
   cirrus, cobweb, cowhide, crop, cytocentrum, deadwood, denier,
   fiber, fibrilla, filament, filamentule, flagella, fork, frond,
   gossamer, hair, hank, horsewhip, kinoplasm, knout, kurbash, lash,
   limb, microbody, microcentrum, mitochondrion, offshoot, pili,
   plasmodesmata, plastid, plastosome, quirt, ramage, ramification,
   rawhide, razor strap, ribosome, runner, sarment, scion, scourge,
   shoot, sjambok, skein, slip, spear, spherosome, spindle fibers,
   spray, sprig, sprit, sprout, stolon, strand, strap, sucker, suture,
   switch, tendril, thallus, thong, thread, threadlet, twig, vacuole,
   web, whip, whiplash