Shadow

A shadow was a dark image made by an object blocking light.

Some species seemed to be made of shadows, such as the Quiet Ones, (AUDIO: Shadow of Death) a shadow creature encountered on Charon, (PROSE: Pluto) and the Elemental Shades. (TV: Love & Monsters)

The Vashta Nerada were not shadows, but a carnivorous swarm that took the shape of shadows to latch onto food sources. Their name literally meant "the shadows that melt the flesh". (TV: Silence in the Library)

The Shadow Kin moved through shadows. They could take the form of pure shadow, but even in their most solid form, could not exist where there was no shadow. (TV: For Tonight We Might Die)

The literal translation of the name "Gallifrey" was "They that walk in the shadows". (PROSE: The Pit)

Shadows and Faction Paradox
During initiation into Faction Paradox, each new Cousin would have a shadow-weapon bound to their shadow. (PROSE: The Book of the War, etc.) Legends heard by the Doctor said that all Faction Paradox members lost their shadows; (PROSE: Unnatural History) indeed, many of the Faction's most experienced agents either had unusually faint shadows or lacked them altogether. However, these shadowless were feared by younger members (PROSE: The Book of the War) and kept separate from those members with shadows. (AUDIO: The Shadow Play)

When the Third Doctor regenerated on Dust, he became infected with the Faction's biodata virus, and when he began to succumb to it in his eighth incarnation, he lost his shadow (PROSE: Interference - Book Two) during a visit to San Francisco in 2002. (PROSE: Unnatural History) The Faction gave him a false shadow so he wouldn't notice. (PROSE: Interference - Book Two) He later lost his shadow again, but regained it following the destruction of Gallifrey and the Faction. (PROSE: The Ancestor Cell)

History
In 1843, Lord Byron lost his shadow during the Clockwork Ouroboros affair. (PROSE: The Book of the War)

In the 2000s, Abaddon's shadow killed every human it touched; when it touched Jack Harkness, he died as well, but the excess life force also killed the demon. (TV: End of Days)

Undated events
Salim tried to steal Ace's shadow, but was stopped by the Seventh Doctor. (AUDIO: The Shadow Trader)

Fu Manchu's son feared that his father would learn to detach his shadow and send it up a drainpipe to strangle him in his sleep. (PROSE: The Beasthouse)

Other references
In a letter sent by the Eleventh Doctor to thank a group of humans for saving him and the world, after he told them to look out for him returning to check up on them, he listed a shadow out of place among the things they should look out for. (PROSE: Hello!)