Danger

Red was the colour for danger on Earth. Throughout the rest of the universe, the colour for danger was mauve. (TV: The Empty Child)

Babies had the ability to sense danger. (TV: The Girl Who Died)

When the TARDIS landed on Wengrol, the First Doctor could tell that there was danger outside from what he saw on the scanner. (PROSE: The Sons of the Crab)

The White Guardian once warned the Fifth Doctor of danger. (TV: Enlightenment)

According to the Valeyard, the Doctor's companions had been placed in danger twice as often as the Doctor. (TV: Mindwarp)

Gilgamesh liked danger, as with it came the chance to gain glory; it also staved off the boredom of life. (PROSE: Timewyrm: Genesys)

The Thirteenth Doctor said that she did not like people being in danger. (TV: Kerblam)

Information from invalid sources
Upon entering the Doctor's TARDIS, Data remarked that time travel was "very dangerous", but that it was okay because he knew what he was doing. (NOTVALID: LEGO Dimensions)