Nanoparticles are small particles that range between 1 to 100 nanometers in size. They are usually defined as particles of matter that are between 1 and 100 nanometers in diameter, although the term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 100 nm in only one dimension. Nanoparticles are usually distinguished from microparticles (1-1000 µm), "fine particles" (sized between 100 and 2500 nm), and "coarse particles" (ranging from 2500 to 10,000 nm), because their smaller size drives very different physical or chemical properties, like colloidal properties and ultrafast optical effects or electric properties.

Nanoparticles occur widely in nature and are objects of study in many sciences such as chemistry, physics, geology, and biology. They are an important component of atmospheric pollution, and key ingredients in many industrialized products such as paints, plastics, metals, ceramics, and magnetic products. The properties of a material in nanoparticle form are unusually different from those of the bulk one even when divided into micrometer-size particles. Many of them arise from spatial confinement of sub-atomic particles (i.e. electrons, protons, photons) and electric fields around these.

Nanoparticles can be composed of natural or artificial polymers and range in size between 10 and 500 nm. They are an important component of nanotechnology produced materials of various types at the nanoscale level. Most nanoparticles are made up of only a few hundred atoms, and their material properties change as their size approaches the atomic scale. Due to their very small size, nanoparticles have a very large surface area to volume ratio when compared to bulk material, such as powders, plate, and sheet. This feature enables nanoparticles to possess unexpected optical, physical, and chemical properties, as they are small enough to confine their electrons and produce quantum effects.