The Collaborative International Dictionary of English v.0.48:

Spore \Spore\ (sp[=o]r), n. [Gr. ? a sowing, seed, from ? to
   sow. Cf. Sperm.]
   1. (Bot.)
      (a) One of the minute grains in flowerless plants, which
          are analogous to seeds, as serving to reproduce the
          species.
          [1913 Webster]

   Note: Spores are produced differently in the different
         classes of cryptogamous plants, and as regards their
         nature are often so unlike that they have only their
         minuteness in common. The peculiar spores of diatoms
         (called auxospores) increase in size, and at length
         acquire a siliceous coating, thus becoming new diatoms
         of full size. Compare Macrospore, Microspore,
         Oospore, Resting spore, Sphaerospore,
         Swarmspore, Tetraspore, Zoospore, and
         Zygospore.
         [1913 Webster]
      (b) An embryo sac or embryonal vesicle in the ovules of
          flowering plants.
          [1913 Webster]

   2. (Biol.)
      (a) A minute grain or germ; a small, round or ovoid body,
          formed in certain organisms, and by germination giving
          rise to a new organism; as, the reproductive spores of
          bacteria, etc.
      (b) One of the parts formed by fission in certain
          Protozoa. See Spore formation, belw.
          [1913 Webster]

   Spore formation.
      (a) (Biol) A mode of reproduction resembling multiple
          fission, common among Protozoa, in which the organism
          breaks up into a number of pieces, or spores, each of
          which eventually develops into an organism like the
          parent form. --Balfour.
      (b) The formation of reproductive cells or spores, as in
          the growth of bacilli.
          [1913 Webster]

WordNet (r) 3.0 (2006):

spore
    n 1: a small usually single-celled asexual reproductive body
         produced by many nonflowering plants and fungi and some
         bacteria and protozoans and that are capable of developing
         into a new individual without sexual fusion; "a sexual
         spore is formed after the fusion of gametes"

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "spore":
   adenovirus, aerobe, aerobic bacteria, amoeba, anaerobe,
   anaerobic bacteria, bacillus, bacteria, bacterium, basidium, bug,
   coccus, cystocarp, disease-producing microorganism, echovirus,
   enterovirus, filterable virus, fungus, germ,
   gram-negative bacteria, gram-positive bacteria, macrospore,
   megaspore, microbe, microorganism, microsporangium, microspore,
   mold, nonfilterable virus, pathogen, picornavirus, planospore,
   protozoa, protozoon, reovirus, rhinovirus, rickettsia, sorus,
   spirillum, spirochete, sporangium, sporocarp, sporocyst, sporocyte,
   sporophore, sporophyte, sporule, staphylococcus, streptococcus,
   swarm spore, trypanosome, vibrio, virus, zoospore, zygospore