- The term "impulse drive" was allegedly derived from "internally-metered pulse drive" (TOS novel: Final Frontier), though in real life, this was probably a retronym.
- This was contrived to explain why the impulse engines on Galaxy-class explorer's primary hulls were off the fore-aft axis of the ship -- and the starship didn't tumble.
Impulse drive was essentially a fusion-powered rocket, usually powered with one or more fusion reactors. These reactors created plasma, who was then exhausted outside the ship, propelling it. (Star Trek: The Stoneship Files: "Dilithium Dance")