Lateral power was an energy resource used for operating a Constitution-class starship.
In 2265, after suffering damage in the galactic barrier, the USS Enterprise returned to the galaxy on lateral power. (TOS: "Where No Man Has Gone Before")
In 2266, Spock accidentally alerted a cloaked Romulan Bird-of-Prey to the Enterprise's location. As a result, they reversed course using lateral power. (TOS: "Balance of Terror")