Any material with particle size less then 100 nanometer (0.1 micron meter) is defined as Nanoparticle.
When material is made into nanoparticles, its reactivity increases. Smaller the particle size, higher the surface area. Nanoparticles have a very high surface area to volume ratio, due to this a higher percentage of atoms (in nanoparticles) can interact with other matter. Therefore Surface Area (measured in Square meters per gram) is most important unit of measure for a nano lubricant. Higher the surface area, higher the lubricity.