Space

"Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. You may think it's a long way down the street to the chemist's, but that's just peanuts to space..."

--The Hitchhiker's Guide to the Galaxy, Introduction

Space is a term referring to two different concepts within physics, the first being the whole of creation, any open area, anywhere. The second is all area in the Universe that exists outside the atmosphere of a planet, usually prefaced as 'outer space'. Space is generally devoid of breathable air; space is full of various gasses and energies, but usually in such low concentration that most living beings cannot survive without assistance in space.

There are certainly exceptions to this rule, ranging from the Time Lord's limited ability to survive in a vacuum to the Hunters' ability to live in outer space indefinitely.

Space travel could be considered the first real step of a race toward sophistication and awareness of other, 'greater' things in the Universe. As ancient as the Time Lords were, they developed space travel even before the Universe was half its present size. (DW: Genesis of the Daleks) This also means that space travel usually pre-dates time travel.

Physics also tells us that space is interwoven with time, and the two should be considered as either one and the same or directly impacting each-other, as most physicists refer to the structure of existence as 'spacetime'. In traditional, Euclidian thinking, space has only three dimensions (height, width, and depth). Using the concept of spacetime, existence then has a 'fourth dimension', creating what is called the 'spacetime continuum', which is often another way of stating the 'universe at large'.

This is, of course, the science-fiction way of looking at things. Generally speaking it's a lot more complicated than that, but whenever one mentions the 'space-time continuum', they are generally referring to the Universe in general.

Additionally, early Doctor Who material was far less technical that it has since become, often using technobabble and inaccurate descriptions under the pretense that most viewers wouldn't understand anyway. For example, in An Unearthly Child, Susan Foreman states that in addition to the three dimensions used in a math problem, you must also use a fourth, time, and a fifth, which she says is space. However, space is made up of the first three dimensions, meaning that her description of the problem was totally wrong.

Space, while vast and mysterious, also acts as a battleground for many races, both lesser and greater. The Time Wars have been fought both across time and across space, with vast armadas from both sides traversing space using various means.

General, broadly-accepted divisions of space help to keep the whole mess under some sort of organization, the widest category being galactic superclusters, the smallest being planetary or solar systems. The Time Lords referred to the galaxy that contained Earth as Mutter's Spiral, though the locals refer to it as the Milky Way. Space-farers tend to refer to whole solar systems by the name of their single inhabited planet, its star or their most populous planet.

While only the greater races of the Universe truly understand all of the wonders, perils, and methods of space, it continues to be a staging-ground for the rest of lifekind.