noun Download 🔗Share 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. Examples : "Sunscreen contains nanoparticles that help protect your skin from the sun's harmful rays. " material science technology physics chemistry environment Chat with AIVocabulary GamePractice Reading