Source : Webster's Revised Unabridged Dictionary (1913)

Allomorph \Al"lo*morph\, n. [Gr. ? other + ? form.] (Min.)
   (a) Any one of two or more distinct crystalline forms of the
       same substance; or the substance having such forms; --
       as, carbonate of lime occurs in the allomorphs calcite
       and aragonite.
   (b) A variety of pseudomorph which has undergone partial or
       complete change or substitution of material; -- thus
       limonite is frequently an allomorph after pyrite. --G. H.
       Williams.

Source : WordNet®

allomorph
     n 1: any of several different crystalline forms of the same
          chemical compound; "calcium carbonate occurs in the
          allomorphs calcite and aragonite"
     2: a variant phonological representation of a morpheme; "the
        final sounds of `bets' and `beds' and `horses' and `oxen'
        are allomorphs of the English plural morpheme"