Nanometers are one billionth of a meter, a metric typically used to measure molecules and scientific building blocks not visible to the human eye. Materials of tens and/or several hundred nanometers in diameter have unique properties, and thus have been widely used in diagnosing and treating various human diseases. One major challenge to use these nano-sized materials is how to deliver them into cells and reach their sites of action.
source https://phys.org/news/2019-08-cellular-engulf-nano-sized-materials.html