dimorphism

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

• noun Biology The existence among animals of the same species of two distinct forms that differ in one or more characteristics, such as coloration, size, or shape.
• noun Botany The occurrence of two distinct forms of the same parts in one plant, as in the juvenile and adult leaves of ivy.
• noun Chemistry & Physics Dimorphic crystallization.

from The Century Dictionary.

• noun In petrography, modifications produced within the magma of an igneous rock by mineralizing or crystallizing agents before its solidification.
• noun The property of assuming or of existing under two distinct forms. Specifically
• noun In crystallography, the property of assuming two distinct crystalline forms not derivable from each other, as by crystallization.
• noun In botany, the occurrence of two distinct forms of flowers or other parts upon the same plant, or upon plants of the same species.
• noun In zoology, difference of form, structure, size, coloration, etc., between individuals of the same species.
• noun In philology, the existence of a word under two or more forms called doublets; thus, dent and dint, fat and vat, church and kirk, exhibit dimorphism developed within English, and card and chart, choir, quire, and chorus, reason, ration, ratio, etc., exhibit dimorphism arising outside of English.

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

• noun (Biol.) Difference of form between members of the same species, as when a plant has two kinds of flowers, both hermaphrodite (as in the partridge berry), or when there are two forms of one or both sexes of the same species of butterfly.
• noun (Crystallog.) Crystallization in two independent forms of the same chemical compound, as of calcium carbonate as calcite and aragonite.

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

• noun biology The occurrence within a plant of two distinct forms of any part.
• noun biology The occurrence in an animal species of two distinct types of individual.
• noun geology A property of certain substances that enables them to exist in two distinct crystalline forms.

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

• noun (biology) the existence of two forms of individual within the same animal species (independent of sex differences)
• noun (chemistry) the property of certain substances that enables them to exist in two distinct crystalline forms