What does "allomorph" mean?

Allomorph: (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.