The Collaborative International Dictionary of English v.0.48:

Isomorphism \I`so*mor"phism\, n. [Cf. F. isomorphisme.]
   1. (Crystallog.) A similarity of crystalline form between
      substances of similar composition, as between the
      sulphates of barium (BaSO4) and strontium (SrSO4). It
      is sometimes extended to include similarity of form
      between substances of unlike composition, which is more
      properly called hom[oe]omorphism.

   2. (Math.) A one-to-one mapping of one set onto another set
      which preserves the relations between the elements of the
      domains of the sets.
      [PJC]

WordNet (r) 3.0 (2006):

isomorphism
    n 1: (biology) similarity or identity of form or shape or
         structure [syn: isomorphism, isomorphy]

The Free On-line Dictionary of Computing (30 December 2018):

isomorphism

    A bijective map between two objects which
   preserves, in both directions, any structure under
   consideration.  Thus a `group isomorphism' preserves group
   structure; an order isomorphism (between posets) preserves
   the order relation, and so on.  Usually it is clear from
   context what sort of isomorphism is intended.

   (1995-03-25)