The Collaborative International Dictionary
Heterology \Het`er*ol"o*gy\, n. [Hetero- + -logy.]
(Biol.) The absence of correspondence, or relation, in type of structure; lack of analogy between parts, owing to their being composed of different elements, or of like elements in different proportions; variation in structure from the normal form; -- opposed to homology.
(Chem.) The connection or relation of bodies which have partial identity of composition, but different characteristics and properties; the relation existing between derivatives of the same substance, or of the analogous members of different series; as, ethane, ethyl alcohol, acetic aldehyde, and acetic acid are in heterology with each other, though each in at the same time a member of a distinct homologous series. Cf. Homology.
Wiktionary
n. A lack of correspondence between parts that reflects a difference in origin.
WordNet
n. (biology) the lack of correspondence of apparently similar body parts