Hyper-impulse drive

From Memory Alpha, the free Star Trek reference.

The hyper-impulse drive was an extension of the impulse drive technology. It presumably allowed for traveling at faster speeds or accelerating faster while remaining below the speed of light. The timeship Aeon was equipped with this drive mechanism, which Henry Starling used when he left Earth in preparation for his trip through time. (VOY: "Future's End, Part II")