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) Kali was the Hindu goddess of death. (COMIC: The Swords of Kali) On Earth, and later human colonies, the skull was a generalised symbol of death, and the idea took this form as an emoji: 💀. (TV: Smile)

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)

As Arboretans existed partly outside of time, when one died, their consciousness travelled back to their birth and they lived their life over again, trying to fix mistakes they had made until they achieved the best life possible. They called this "the path of perfection". (PROSE: Festival of Death)

The Twelfth Doctor was willing to accept death on at least two occasions, each time wondering why he can't be allowed his "sleep." (TV: Heaven Sent, Twice Upon a Time)

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 and the original template was restored. All across the world, those who were unable to die yet should have died, including Geraint Cooper and a little girl, all took one final collective breath before dying. For them, the return of mortality was a mercy for them. In the case of others, like the Mother and the Cousin who were responsible for the whole thing, it came back with a vengeance. (TV: The New World, The Blood Line)

Biology of death
In the final stages of mental shutdown, when physical shutdown would commence, brainsteam death was a sign closely followed by complete respiratory collapse. Soon, liver failure and renal collapse would signal general systemic failure, leading to death within an hour. (PROSE: Anachrophobia)

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)