1. (n.) The property of crystallizing in three forms fundamentally distinct, as is the case with titanium dioxide, which crystallizes in the forms of rutile, octahedrite, and brookite. See Pleomorphism.

2. (n.) The coexistence among individuals of the same species of three distinct forms, not connected, as a rule, by intermediate gradations; the condition among individuals of the same species of having three different shapes or proportions of corresponding parts; -- contrasted with polymorphism, and dimorphism.