Space

Space was a word used to refer to two different concepts relating to the universe. One referred to dimensions: space being described by Susan Foreman as the fifth dimension. (TV: "An Unearthly Child") Space was also defined as the vacuum in the universe, outside of a planetary body's atmosphere. (TV: Oxygen) This was often known as "outer space". (TV: The Runaway Bride)

According to the Twelfth Doctor, space was bent, so Earth was in any particular direction one chose to look in. (TV: Smile)

The word space was often attached to other words to denote otherworldliness. Though the Twelfth Doctor insisted that people didn't add the word to things like restaurant, champagne or hat to make terms like space restaurant, Clara Oswald pointed out space suit and space pirate as examples to the contrary. (TV: Sleep No More)

The Twelfth Doctor maintained that planets themselves were not included in outer space, that he was not "from space" as he came from a particular planetary body, Gallifrey. Most people, he argued, came from a planet, rather than from the vacuum of space itself. (TV: The Pilot)

The physics of space
Space was not as cold as -271.3°C. (TV: Extremis) There was no pressure in space, nor, of course, oxygen. (TV: Oxygen; AUDIO: The Cold Equations) There was also little, if any, gravity. (TV: Underworld, The Beast Below, et al.)

Space/vacuum exposure
Most lifeforms could not survive in the vacuum of space for very long, at least not without a spacesuit, and a good supply of oxygen, among other necessities. (TV: The Moonbase, Oxygen; AUDIO: The Cold Equations)

According to the Twelfth Doctor, since there was no pressure in a vacuum, one's lungs would explode if they tried holding their breath. Blood vessels would also rupture, and exposed areas of the body would swell up. The boiling point of water was much lower in a vacuum, so one's sweat, saliva and the fluid around their eyes would also boil.

Within 15 seconds, a humanoid exposed to the vacuum of space would lose consciousness, as oxygen bubbles would form in their blood. Within 90 seconds, they would be dead. (TV: Oxygen)

While Time Lords could remain alive in the vacuum of space for longer than humans, (TV: Four to Doomsday, The Doctor, the Widow and the Wardrobe, GAME: TARDIS) extended exposure could lead to severe consequences. Prolonged exposure to save his friend, Bill, blinded the Twelfth Doctor, though he did survive. (TV: Oxygen)

Krynoid pods survived exposure to the vacuum of space by hibernating during voyages through space. (TV: The Seeds of Doom)

Wirrn lived in and travelled through space. They could live without fresh oxygen for several years, but their breeding colonies were terrestrial. (TV: The Ark in Space)

The Rutan Host could travel through space without any effects. (PROSE: Shakedown)