What is Immortality?

DESOLATION. That's all the Doctor felt right now as the weight of the only Time Lord left in existence, as well as the genocide of the Daleks. He truly felt alone in the universe, for anything he did only made the now present better, and anywhere, anytime would be horrible unless he put a stop to it. People died, planets destroyed, faiths and beliefs evaporated. He simply saw no use in interfering anymore, for no matter what he did, he would forever be, ALONE.

The weight of an almost infinite memory is burdening, constantly remembering things in every single moment, commemorating lives that have been lost, reliving deaths and births, continually recalling mysteries that should never be told and wisdom that should never spoken. It took most of his willpower not to sob from all the agony his memories arose in him, causing him to put back on the facade he hated to wear, the one that caused him to seek out adventure in the first place. The emotion of eternal calm. He was now perpetually apathetic, due to knowing everything in the universe and able to solve (almost) any problem in the universe, except one.

What is Immortality?