The Collaborative International Dictionary of English v.0.48:

Diatom \Di"a*tom\ (d[imac]`[.a]*t[o^]m), n. [Gr. dia`tomos cut
   in two. See Diatomous.]
   1. (Bot.) One of the Diatomace[ae], a family of minute
      unicellular Alg[ae] having a siliceous covering of great
      delicacy, each individual multiplying by spontaneous
      division. By some authors diatoms are called
      Bacillari[ae], but this word is not in general use.
      [1913 Webster]

   2. A particle or atom endowed with the vital principle.
      [1913 Webster]

            The individual is nothing. He is no more than the
            diatom, the bit of protoplasm.        --Mrs. E. Lynn
                                                  Linton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

diatom
    n 1: microscopic unicellular marine or freshwater colonial alga
         having cell walls impregnated with silica