Hyperspace

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

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

Hyperspace permitted communication over great distances. Through use of an ionic vibrator, Prentice, her mind in shock from a fall, was able to hear the voice of Zex, even though he was five hundred light-years away. The Master used the same device to send his thoughts into hyperspace and contact Zex. (DWA: Listen - The Stars)

The Third Doctor materialised in front of a ship, forcing it to pull out of hyperspace. (DW: Frontier in Space)

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