Ultrafine particle

Ultrafine particle, n–in nanotechnology, a particle ranging in size from approximately 0.1 micrometer (100 nanometers) to 0.001 micrometers (1 nanometer).

DISCUSSION–The term is most often used to describe aerosol particles such as those found in welding fumes and combustion by-products. The length scale may be measured by a particle's geometric, aerodynamic, mobility, projected-area, or hydrodynamic dimension.

Source:
ASTM International. E 2456-06. Terminology for Nanotechnology