A postern door, also known as a back door, is a secondary entrance to a building or property. Other synonyms for postern door include rear door, side door, or secret door. These doors are often used as a discreet way to enter or exit a building without attracting attention. In some cases, a postern door refers to a small, hidden door located within a larger door or wall. This type of door is commonly seen in medieval castles and fortifications. No matter the term used, a postern door provides an alternative access point that can offer convenience and protection.