Heart

The Heart was an organ in many lifeforms. Humans had a single heart whilst Time Lords usually gained their second heart after their first regeneration. (MA: The Man in the Velvet Mask)

The Great Vampire, lesser vampires and Haemovores could only be killed by a stake through the heart. (DW: State of Decay, The Curse of Fenric)

Alternative definitions
Heart can also refer to the physical or ideological centre of a location; Looms were the heart of the Great Houses (NA: Lungbarrow), or the Kandy Man's kitchen which was at the heart of Terra Alpha's governance system. (DW: The Happiness Patrol)

Heart can also refer to the change in emotional standing in an individual ie them having a "change of heart" such as Maldak saving the Governor and Peri Brown from execution on Varos (DW: Vengeance on Varos) or Alan Jackson changing his mind to move away from Sarah Jane Smith. (SJA: The Lost Boy)