Nanoparticle

"A particle with at least one dimension smaller than 100 nm including engineered nanoparticles, ambient ultrafine particles (UFPs) and biological nanoparticles."¹

"Nanoparticles are an obvious form of engineered nanomaterial presenting a significant exposure potential, because they can be readily deposited in the lungs or on the skin, and potentially translocate within the body."²

"In nanotechnology, a sub-classification of ultrafine particle with lengths in two or three dimensions greater than 0.001 micrometer (1 nanometer) and smaller than about 0.1 micrometer (100 nanometers) and which may or may not exhibit a size-related intensive property."³

"Particles having a diameter between 1 and 100 nm.  Nanoparticles may be suspended in a gas (as a nanoaerosol), suspended in a liquid (as a colloid or nanohydrosol) or embedded in a matrix (as a nanocomposite).  The precise definition of particle diameter depends on particle shape as well as how the diameter is measured.  For instance, carbon fullerenes represent nanoparticles with identical dimensions in all directions (spherical), whereas single-walled carbon nanotubes (SWCNTs) typically form convoluted, fibre-like nanoparticles with a diameter below 100 nm."⁴

1. G. Oberdörster, A. Maynard, K. Donaldson, et al, “Principles for characterizing the potential human health effects from exposure to nanomaterials: elements of a screening strategy,” Particle and Fibre Toxicology 2005, 2:8, § 1.0.