A doorway in Halachah is a space that consists of two doorposts and a lintel.

One has to put up mezuzahs by every doorway in their house that is used for entering and leaving even if one door is used more than the other.  A doorway that is not used for entering and leaving and was built just for beauty or different reason does not need a mezuzah. A door like an emergency door that is used only if there will be an emergency, does not need a mezuzah. A door that is not being used now because it is being blocked by a closet or something similar, does not need a mezuzah.