A crystallite is a domain of solid-state matter that has the same structure as a single crystal.
Solid objects that are large enough to see and handle are rarely composed of a single crystal, except for a few cases (gems, silicon single crystals for the electronic industry, certain types of fiber, and single crystals of a nickel-based superalloy for turbojet engines). Most materials are polycrystalline, i.e. made of a large number of single crystals - crystallites - "stuck" together. The crystallite size can vary from a few nanometers to several millimeters.
Metallurgists often refer to crystallites as "grains"; thus, the boundary between two crystallites is the grain boundary , and fracture can be an intergranular fracture or a transgranular fracture . But there is an ambiguity with powder grains: a powder grain can be made of several crystallites. Thus, the (powder) "grain size" found by laser granulometry can be different from the "grain size" (in fact: crystallite size) found by X-ray diffraction (e.g. Scherrer method), by optical microscopy under polarised light or by scanning electron microscopy (backscattered electrons).
Therefore the term "crystallite" is more precise. However, the term "crystallite boundary" is not used, in favor of the traditional "grain boundary".
See also
crystal, crystallography