Living metal was a metal which was able to grow.
In an example of a naturally-occurring living metal lifeform, the Magnedons, one of the lifeforms native to Skaro, was a crocodilian reptile with living metal flesh. It was, as a result, magnetic. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)
The Founders of the Time Lords utilised the living metal validium to construct Nemesis. (TV: Silver Nemesis) The outer plasmic shells of Battle TARDISes were living metal, able to morph into shields or any number of predetermined gun emplacements. (PROSE: Engines of War) The architectural reconfiguration system of the Doctor's TARDIS was also built from a type of living metal. (TV: Journey to the Centre of the TARDIS)