Ion propulsion

From Memory Alpha, the free Star Trek reference.

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

Ion propulsion is a highly-advanced form of interstellar propulsion. It uses advanced ion power and leaves behind an ion trail. It was hypothesized by the Federation before 2268, but was beyond their capability.

In 8000 BC, the Eymorg civilization on Sigma Draconis VI had already developed ion propulsion.

In 2268, the USS Enterprise witnessed the use of ion propulsion by an ancient Eymorg starship, much to the surprise of Chief Engineer Montgomery Scott. Knowledge of the Eymorg's ion propulsion was likely lost when the Controller was destroyed. (TOS: "Spock's Brain")

Some Akritirian starships use ion-based propulsion, fueled by paralithium. (VOY: "The Chute")