An Organ was a complex system in an organism which typically performed a vital process which kept the organism alive.
A heart was an organ used to pump blood through the body. [source needed] Time Lords had two hearts. (TV: Spearhead from Space, et al)
A stomach was an organ in the digestive tract use by many species to digest food [source needed] and some by others as a form of communication. (TV: Flatline)
Time Lords possessed an organ called an exo-spleen. (TV: The Tsuranga Conundrum)
A lung was an organ primarily used for respiration. (TV: Smith and Jones) Though some species could uses them aggressively. (TV: Boom Town)