English definition of “extension”
/ɪkˈstenʃən/ noun [C]
PART OF A BUILDING › a new room or rooms that are added to a building: You could build an extension onto the back of the house.
EXTRA TIME › extra time that you are given to do or use something: You might be able to get an extension on your visa.
TELEPHONE › a telephone that is connected to the main telephone in an office or other large building: Call me on extension 213.