What is the term used to describe a portion of a building below the natural or finished grade?

The term used to describe a portion of a building that is located below the natural or finished grade is "basement." A basement is typically a space that is partially or entirely below the ground level, providing various functionalities such as storage, living space, or mechanical equipment housing. This distinction is crucial in architectural design and building codes, as basements can have specific regulations regarding ventilation, ingress and egress, and water drainage to ensure safety and habitability.

In contrast, other options refer to different parts of a building. A cellar, while similar to a basement, often specifically indicates a space primarily used for storage, particularly of wine or other perishable goods, and may not be designed for regular human occupancy. An attic is a space found directly under a roof, typically used for storage or sometimes converted into living space, but it is never below grade. The ground floor refers to the level of the building that is at the same height as the surrounding ground, not below it. These distinctions are essential for understanding building terminology and their implications in construction and design.