Spacetime, or space-time, was the mathematical combination of time and space, combined into the space-time continuum.

Parallel dimensions ran along spacetime, side by side like grooves on a record. (AUDIO: Aquitaine)

Romana II explained that when connected to the Matrix, the user was outside of real spacetime, "alone on the astral plane." (AUDIO: The Inquiry)

Interstitial time was an envelope of non-space, non-time, underlying the "real" universe and surrounding and separating every space-time event. The inside of the Doctor's TARDIS was considered outside of the real universe, (PROSE: Falls the Shadow) described as not being in local spacetime. Vilal vortex ships existed in the time vortex, on the "edge" of spacetime. (AUDIO: Quicksilver) The granularity of space-time allowed the Master to utilise interstiality and pull objects through time. (TV: The Time Monster, PROSE: The Quantum Archangel)

The Watcher took the TARDIS with Adric and Nyssa out of "time and space" before heading to the Pharos Project on Earth. Presumably, that was done to avoid the entropy wave destroying the universe. (TV: Logopolis)

The Neverwhen existed outside of normal spacetime. (AUDIO: The Neverwhen)

Behind the scenes

In Iris Wildthyme and the Unholy Ghost, the term "tempero-spatial" was used, which in the real world is a genuine term to describe spacetime, but this definition was never given within the story.