Nanoparticle
Definition
A particle with at least one dimension between roughly 1 and 100 nanometres, at which size its behaviour stops matching the bulk material.
The size range is a convention rather than a physical boundary, but a defensible one: below about 1 nm you are describing individual molecules, and above about 100 nm the surface-to-volume ratio has dropped far enough that most materials behave as bulk.
The practical consequence is that a nanoparticle of a familiar substance can be a genuinely new material. Bulk gold is inert; 20 nm gold catalyses reactions at room temperature.
Where this comes up
Articles that define nanoparticle in context.
