Space

Space was a word used to refer to two different concepts relating to the universe. One referred to dimensions; space was linked to time, with space being three dimension and time the fourth. (DW: An Unearthly Child) 'Space' was also defined as the vacuum in the universe, outside of a planetary body's atmosphere. This was often known as "outer space".

Space travel
Space was a vacuum and so a hostile environment as. Many species required a form of spacecraft to travel through space. Some species such as Time Lords could survive for a time in the vacuum of space, though they still needed to breathe oxygen. (DW: Four to Doomsday, The Doctor, the Widow and the Wardrobe)

Development of the ability to travel through space was regarded as one of the first real steps species took in their planetary and species' development. The Time Lords developed space travel when "the universe was half its present size." (DW: Genesis of the Daleks) Humanity's first sojourn into exploring space began in the mid-20th century with the launch of several satellites. (EDA: Alien Bodies)

Biological abilities
Kitlings were a species able to travel through space (and time) without any technology. (DW: Survival, NA: First Frontier)

Likewise, the Eight Legs could also transport themselves through space. (DW: Planet of the Spiders)

Space/vacuum exposure
Krynoid pods survived exposure to the vacuum of space by hibernating during voyages through space. (DW: The Seeds of Death)

Wirrn also survived in and travelled through space. They could live without fresh air for several years, but needed to return to a planet for food or to breed. (DW: The Ark in Space)

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