A lung is an organ of respiration. Lungs are a collection tiny air sacs called alveoli which make a huge surface area where gases are exchanged through the thin walls into the bloodstream.
In April 2376 the Benzite Monyodin was exposed to a cloud of chemical polymer gas. The gas liquefied the alveoli in his lungs which led to his death. (DS9 novel: Avatar, Book One)