a part of a building consisting of rooms that are partly or completely below the level of the ground
They turned their basement into a game room.
The basement of the old house was a dark and musty space, used primarily for storage and housing the furnace.
They converted the basement into a cozy home theater, complete with a big screen and comfortable seating for movie nights.
The basement served as a refuge during the tornado, providing a safe place for the family to take cover from the storm.