Death

Death was the cessation of life. The Ninth Doctor told Rose that "everything dies". (TV: The Parting of the Ways) Every single death caused a tiny fracture in reality. (TV: The Woman Who Lived) In 2011, three hundred thousand people died on Earth every day. (TV: The New World)

The Eternal, Death, was a personification of the concept. (PROSE: Timewyrm: Revelation)

When discussing the idea of a "planet ruled by zombies", the Fourth Doctor remembered the quote that "the living are just the dead on vacation." (TV: Destiny of the Daleks) Voicing her observations on the human lifespan, which she found "hilarious[ly]" short, stated that human beings were born dying, remarking "You know from the minute you slop out, you're rotting, decaying." (TV: Death in Heaven)

In 2011, for several months following the Miracle Day, every human on Earth simply stopped dying. It was caused when the Three Families fed the immortal Jack Harkness' blood into the Blessing, which in turn relayed the template to everyone on the planet in seconds. Death was restored when Jack's mortal blood was fed into the Blessing; though it was a mercy for some, like Geraint Cooper and a little girl, for others it came back with a vengeance, like the Mother and the Cousin. (TV: The New World, The Blood Line)

Kali was the Hindu goddess of death. (COMIC: The Swords of Kali)

Alternate timelines
In one alternate timeline, the Doctor died during the time war with the Enemy and his body was auctioned on 21st century Earth for its biodata. (PROSE: Alien Bodies)