Stoichiometry

Stoichiometry ( in Greek - measuring element) takes account of quantity of elements involved in a chemical reaction and is based upon the principal of mass conservation, which applies to chemical reactions that by definition do not affect the mass of reactants nor transmute into other elements. The word is also used for molar proportions of elements that are discrete numbers.
Non-stoichiometric compound refers to the ratio of atoms not being an integer, such as Vanadium Oxide varieties from VO 0.79 to VO 1.29, and others TiOx, NixO, UOx and LiWO3. extrinsic defects are introduced into crystal through impurities, that is by adding dopants. For example if ClNa is heated in an environment of sodium vapour, the Na+ ion changes to Na(1+x) Cl. Sodium ions now moved to the crystal and occupy cation sites and leave same quantity of unoccupied anion sites behind which then will be occupied by anion vacancies. This state of solid is non-stoichiometric mix.
Refs.

http://www.chem.ox.ac.uk/vrchemistry/solid/Page17.htm
www.wikipedia.com