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)