Mortality

Mortality was the concept that an individual's life would end with death. (TV: The Age of Steel)

All species were originally immortal during the Dark Times, but the Kotturuh set out on their "Great Task," going to planets and setting lifespans. At the end of their lifespan, an individual would die. (PROSE: The Guide to the Dark Times, The Dawn of the Kotturuh)

The Tenth Doctor believed that, without mortality, there would be nothing to strive for. (TV: The Age of Steel)