Nanotechnology was a field of science and technology involving devices so small that the naked eye could not see them.
In 2153, scientists of the Arctic Archaeology Team accessed a medical report from the nanotechnology database while they examined two Borg drones. (ENT: "Regeneration")
Denobulans were known to have experimented with nanotechnology by the early 2150s. (ENT: "Regeneration")
The Borg Collective was infamous for their use of nanotechnology to advance their goals, specifically assimilating new members and technologies with nanoprobes. (VOY: "Scorpion")
A similar method was developed by Control after it went rogue in the late 2250s, as a way of taking over flesh beings. It had previously relied on fabricated holographic communications, but required more freedom to operate. The nanobots used by Control used ferromagnetic materials. (DIS: "Perpetual Infinity", "Through the Valley of Shadows")
The Federation Starfleet also commonly used nanotechnology, such as in isolinear circuitry. In 2366, Wesley Crusher was experimenting with nanites, small robotic devices in the field of nanotechnology. (TNG: "Evolution")
Major Kira Nerys once delayed Vedek Winn's ship and claimed it was to inspect for nanotechnology. (DS9: "The Collaborator")