storm door

Short definition from Wiktionary

Noun

1. 1
   A door normally leading to a storm cellar, cellar or a basement, for protection from a tornado or a violent storm.
2. 2
   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.
3. 3
   Archaic form of storm door.