An extra outer door for protection against severe weather or winter WordNet 3.0 & Open English WordNet

A storm door is a type of door that is installed in front of an exterior access door to protect it from bad weather and allow ventilation. Wikipedia

A secondary door placed exterior to the primary door, to better weatherproof an entrance. Those storm doors that are also screen doors can be used for ventilation purposes when the primary door remains open. Typically found on front doors, rear doors, side doors, balcony doors. Wiktionary