Yoyodyne pulse fusion, or YPS pulse fusion, was a type of engine utilized by DY-500 class starships as early as the early 22nd century. (TNG: "Up The Long Ladder")
The propulsion systems used aboard 23rd century Template:ShipClass starships were YPS pulse fusion. (Star Trek Generations)
Background
This "drive type" was described in an okudagram that appeared in the episode and may have been designed or constructed by the group later described as Yoyodyne Propulsion Systems.
The use of the term "Yoyodyne" is a "in-joke" reference to the 1984 cult classic science fiction film The Adventures of Buckaroo Banzai Across the Eighth Dimension!, which in turn took the name from Thomas Pynchon's novels The Crying of Lot 49 and V.