1. a floor or level below the basement of a building

Examples

• The old hospital had a subbasement that housed the morgue.
• Storage rooms were located in the subbasement of the office building.
• The elevator descended to the subbasement parking garage.
• Mechanical equipment was installed in the subbasement to save space.
• The subbasement flooded during the heavy rains last spring.
• Access to the subbasement required a special security card.