Definition of "nanoparticle"

Nanoparticles of various sorts are already found in products like sunscreen, paint and inkjet paper. More exotic varieties offer promise in medicine for sensitive diagnostic tests and novel treatments: the detection of Alzheimer's disease by finding a protein in spinal fluid, for instance, or nanoparticles that heat up and kill cancer cells.

2005 February 22, Kenneth Chang, “Tiny Is Beautiful: Translating 'Nano' Into Practical”, in The New York Times‎