isomorphism

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Biology Similarity in form, as in organisms of different ancestry.
• noun Mathematics A one-to-one correspondence between the elements of two sets such that the result of an operation on elements of one set corresponds to the result of the analogous operation on their images in the other set.
• noun A close similarity in the crystalline structure of two or more substances of similar chemical composition.

from The Century Dictionary.

• noun In biology, the state or condition of being different in ancestry, but alike in appearance. See isomorphic, 2 .
• noun In group-theory, the state or character of being isomorphic.
• noun A similarity of crystalline form: as, between substances of analogous composition or atomic proportions, as the members of a group of compounds like the sulphates of barium, strontium, and lead
• noun between compounds of unlike composition or atomic proportions.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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 homœomorphism.
• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Similarity of form

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (biology) similarity or identity of form or shape or structure