Space-time continuum

The space-time continuum was a term for the continuous set of coordinates created by the whole of space added to the whole of time. On Earth in the 20th century, the acceptance of Albert Einstein's Theory of Special Relativity demonstrated that time was not a constant. The concept of the space-time continuum — with time as an equal dimension — replaced the older idea of three spatial dimensions plus time.

The First Doctor stated that the "fourth dimension", as humans understood it, was time. (TV: An Unearthly Child) N-Space, which was the prime universe according to the Doctor, was comprised of eleven dimensions, including those comprising the space-time continuum. (PROSE: Parasite) The Doctor's TARDIS herself stated that she was an eleven dimensional being that existed across all space and time. (TV: The Doctor's Wife)

By the time continuum of the 20th century, Gallifreyan civilization existed far in the past. (PROSE: Goth Opera)