Source : Webster's Revised Unabridged Dictionary (1913)

Diatom \Di"a*tom\, n. [Gr. ? 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.

   2. A particle or atom endowed with the vital principle.

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

Source : WordNet®

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