Nanite

A nanite is a microscopic robotic device and a form of nanotechnology.

A nanite is built by manipulating atoms and contains gigabytes of computer memory. It is small enough to enter living cells and can be programmed to do numerous tasks. Nanites are used by the Federation for medical purposes and are designed to work inside nucleii during cellular surgery. When they are not used, nanites are stored in a non-functional state. When necessary nanites can be destroyed with a burst of high-level gamma radiation.

Nanites consist of several components with different functions. Nanites used in the 24th century are composed of a forward propulsion block, an Oneamisu receiver, a Ranma ½ gyro block, a hex growth stack, Otomo calibration, two reverse flow paddles, one cell membrane sensor and manipulator each, a telemetry antenna, a forward motion sensor, a crystal articulator, logic circuits arranged from A-F and G-L, a T-cell antireceptor and an emergency dissolve system.

The Borg use a similar technology called nanoprobes.

Thanks to Memory Alpha for the information.