Definition of Crystallite
Crystallite is microscopic or a very small crystal that is formed during the cooling down of several materials. The direction of the crystallites is totally random. Crystallites are also named grains. The regions where the two crystallites meet are known as grain boundaries. They are generally only a few nanometers in width.
Polycrystals are defined as those solids which are composed of a large number of crystals having different sizes and directions. Most of the inorganic solids are polycrystalline. The structure of a single crystal is well defined and ordered and its lattice is continuous.