A lung was an organ used for respiration.
Instead of large lungs, Gallifreyans had a series of pulmonary tubes parallel to the lymphatic system. This made them buoyant, allowing them to swim with ease. (PROSE: Island of Death) They could also survive longer without much oxygen, past the point where a human would be unconscious. (TV: The Ark in Space, Smith and Jones, Mummy on the Orient Express, PROSE: Legacy)
President Ronald Reagan was the victim of an assassination attempt by John Hinckley Jr in 1981, receiving a bullet in his lung. (PROSE: The Assassin's Story)
Female Raxacoricofallapatorians could release poison with a dart from their finger. As a "final resort", the excess poisons were exhaled through the lungs. (TV: Boom Town)
Silurians had a greater lung capacity than humans. (TV: Deep Breath)
Nardole had human lungs, which the Twelfth Doctor got for cheap (TV: The Pyramid at the End of the World) when reassembling him. (TV: The Return of Doctor Mysterio)