What does allotropism or allotropy mean?
The definition of allotropism and allotropy
Al*lot”ro*pism, Al*lot”ro*py, n.
Etym: [Gr. allotropie.] (Chem.)

The property of existing in two or more conditions which are distinct in their physical or chemical relations.

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.