The Collaborative International Dictionary of English v.0.48:

Allotropism \Al*lot"ro*pism\, Allotropy \Al*lot"ro*py\, n. [Gr.
   ? other + direction, way, ? to turn: cf. F. allotropie.]
   (Chem.)
   The property of existing in two or more conditions which are
   distinct in their physical or chemical relations.
   [1913 Webster]

   Note: Thus, carbon occurs crystallized in octahedrons and
         other related forms, in a state of extreme hardness, in
         the diamond; it occurs in hexagonal forms, and of
         little hardness, in black lead; and again occurs in a
         third form, with entire softness, in lampblack and
         charcoal. In some cases, one of these is peculiarly an
         active state, and the other a passive one. Thus, ozone
         is an active state of oxygen, and is distinct from
         ordinary oxygen, which is the element in its passive
         state.
         [1913 Webster]

WordNet (r) 3.0 (2006):

allotropism
    n 1: the phenomenon of an element existing in two or more
         physical forms [syn: allotropy, allotropism]