Ion propulsion


 * You may also be looking for the sub-light ion drive.

Ion propulsion was a highly-advanced form of interstellar drive considered by Starfleet to be only theoretical as of 2268.

In that year, the starship USS Enterprise (NCC-1701) encountered a vessel from Sigma Draconis VI which utilized ion propulsion. The crew eventually discovered that the technology had been developed in the distant past, and that its users could only operate it for short periods of time while under the influence of the Teacher. 

Some Akritirian starships used ion-based propulsion, fueled by paralithium.