Life force

Life force was basically the "soul" of a living being. (TV: The Doctor, the Widow and the Wardrobe)

Many creatures had the ability to drain another's life force like Fendahl, (PROSE: The Left-Handed Hummingbird) or the mutated Lazarus, which could leave the victim looking like a dried husk. (TV: The Lazarus Experiment)

The First Doctor had his life-force drained, which triggered his regeneration into his second incarnation. (TV: The Tenth Planet)

Abaddon fed off the life-force of living creatures, completely draining it from them. Jack Harkness, being a fixed point in time, had an over abundance of life-force. When Abaddon attempted to steal Jack's, there was too much, and Abaddon was destroyed. (TV: End of Days)