A mesocrystal is a material structure composed of numerous small crystals of similar size and shape, which are arranged in a regular 
periodic pattern. It is a form of oriented aggregation, where the small crystals have parallel crystallographic alignment but are spatially separated.
When the sizes of individual components are at the nanoscale, mesocrystals represent a new class of nanostructured solids made from crystiallographically oriented nanoparticles. The sole criterion for determining whether a material is mesocrystal is the unique crystallographically hierarchical structure, not its formation mechanism.