A nanocrystalline material is a polycrystalline material with a crystallite size of only a few nanometers. These materials fill the gap between amorphous materials without any longe range order and crystalline materials with a clear three dimensional long range order.
X-ray diffraction is commonly used to determine the crystallinity of a material. When the crystallites of a polycrystalline material become very small, the diffraction signals become very wide. A material can be called nanocrystalline when the diffraction signals are too sharp to originate from an amorphous material but too wide to determine the crystal structure. The upper limit for the crystallite size of nanocrystalline materials is about 5 nm, the lower limit will be close to 0.5 nm.