A Chosen One is a person chosen by the Fairies to eventually become a fairy. The Fairies protect the person, sometimes by killing the attacker.
In 1909, one of Jack Harkness's soldiers accidentally killed a Chosen One in Lahore. In retaliation, the Fairies killed all his men, sparing only Jack.
In 2007, Torchwood Three was unable to stop the Fairies from claiming Jasmine Pierce as their Chosen One. (TW: Small Worlds)