Hyperspace

Hyperspace existed as a separate dimension to normal space. According to the Doctor it was an absurdity. It was only supposed to be a theory. (DW: The Stones of Blood)

Yet it existed, and could allow ships to travel great distances while in another type of space which allowed for an alternative to faster than light drives. They would disappear from normal space, enter hyperspace and return. (DW: Nightmare of Eden) Icarons can be used to allow a ship make the transition between normal space and hyperspace. (NA: Original Sin)

By the 25th century, humans had the ability to travel in hyperspace and such journeys had become common place and commercial. (NSA: Judgement of the Judoon)