This paper addresses the question of “how large is large enough for a crystallite to be a three dimensional Bravais lattice?” Based on the premise that the ratio of the bulk volume to that of the volume of the unit cell for a given material should determine the transition to a large molecule, it is proposed that for values of the ratio < 106crystallites can be considered as large molecules. The universality of this criterion has been established by examples from magnetism, optics, ferroelectrics and superconductivity. It is expected, due to the wide variety of materials considered, that other classes of inorganic materials would also obey this rule leading to a quantitative definition for a nanocrystal or nanostructure. © 2003 American Institute of Physics