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 left 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)