A greatcoat was a type of overcoat, especially associated on Earth with the First and Second World Wars. Such coats were almost exclusively worn by men. They easily fit over even full dress uniforms. Typically made of a heavy fabric, such as wool, they extended down to the mid-calf, to provide warmth over almost the entire body. They were distinguished by this exceptional length, prominent pockets, substantial collars and large buttons. Typically, greatcoats also had a strip of cloth with decorative buttons on the back, at roughly waist level. This strap across the back naturally gathered the cloth across the back to provide a characteristic pleat.
Worn by the Doctor
The Eleventh Doctor wore an army green greatcoat when helping River Song in Berlin during 1938, (TV: Let's Kill Hitler) during a misadventure in the Two Streams Facility, (TV: The Girl Who Waited) and when escaping death. (TV: The Wedding of River Song) He also wore a black greatcoat when decoying Kahler-Tek for Kahler-Jex's escape him in Nevada during 1870. (TV: A Town Called Mercy)
The Second Doctor donned a greatcoat as a disguise to escape from the War Lord's central headquarters. (TV: The War Games)
The Eighth Doctor wore a greatcoat when dealing with Vortex Wraiths. (PROSE: The Slow Empire)
Worn by Companions
Jack Harkness wore a navy blue greatcoat. Adopted when the Time Agent tried to perpetrate a con in the middle of the London Blitz, Harkness continued to wear the coat and many copies for the many long centuries after he became effectively immortal. (TV: The Empty Child and others) At some point, he came to view it as absolutely essential to his identity, and went to great lengths to ensure that he always had a greatcoat to wear. (TV: Immortal Sins)
Jamie McCrimmon and Zoe Heriot wore greatcoats as disguises. (TV: The War Games)
Worn by Others
An early incarnation of Iris Wildthyme once wore a greatcoat. (PROSE: Hospitality)
Behind the scenes
The Second Doctor names the object in The War Games. In Torchwood, it largely goes unnamed. However, it is the subject of puns, such as when Ianto Jones calls it a "great coat" after one of his initial encounters with Jack Harkness. (TV: Fragments)