Wikipedia, Aug. 8, 2005 ("a nanoparticle is a microscopic particle whose size is measured in nanometers. Often such particles in nanoscale are used in biomedical applications acting as drug carriers or imaging agents.")

Britannica online, July 17, 2020 (Nanoparticle, ultrafine unit with dimensions measured in nanometres (nm; 1 nm = 10⁻⁹ metre). Nanoparticles exist in the natural world and are also created as a result of human activities. Because of their submicroscopic size, they have unique material characteristics, and manufactured nanoparticles may find practical applications in a variety of areas, including medicine, engineering, catalysis, and environmental remediation)

What are nanoparticles?, via TWI website, viewed July 17, 2020 (A nanoparticle is a small particle that ranges between 1 to 100 nanometres in size. Undetectable by the human eye, nanoparticles can exhibit significantly different physical and chemical properties to their larger material counterparts. The definition given by the European Commission states that the particle size of at least half of the particles in the number size distribution must measure 100 nm or below. Most nanoparticles are made up of only a few hundred atoms) https://www.twi-global.com/technical-knowledge/faqs/what-are-nanoparticles

Journal of colloid and interface science, v. 521 (July 2018): p. 69 (article title: Nano-particle dynamics during capillary suction; nanoparticles)

Journal of environmental management, v. 196 (1 July 2017): p. 297 (article title: Engineered nano particles; abstr.: a wide variety of nano particles (NPs) are ubiquitously dispersed in ecosystems)

Springer handbook of nanotechnology, 2017: p. 643 (article title: Nano-particles for biomedical applications; Nanoparticles (NPs) are extremely small particulates with an average size that ranges from a micron or less to a few nanometers. The large majority of NPs necessitate nanotechnology methods for their production)