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.
The Second Doctor, Jamie McCrimmon and Zoe Heriot, along with several resistance members, donned greatcoats as disguises to escape from the War Lord's central headquarters. (TV: The War Games) Much later, in his eighth incarnation, the Doctor again wore a greatcoat — this time as a replacement for his own, destroyed clothing. (PROSE: The Slow Empire)
Jack Harkness also wore a greatcoat. Adopted when the Time Agent tried to perpetrate a con in the middle of the London Blitz, Harkness continued to wear the coat — or at least others like it — 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)
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)