A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres in diameter. Wikipedia

An object between 1 and 100 nanometers that behaves as a whole unit Wikipedia Disambiguation

Particle with size less than 100 nm Wikidata

Any microscopic particle less than about 100 nanometers (nm) in diameter. In aerosol science, the term is often reserved for particles less than 50 nm in diameter; the term "ultrafine particles" is used for particles less than 100 nm in diameter. Wiktionary

Microscopic particle. Wiktionary (translation)