The term used to describe the area under the lowest elevated floor of a building, enclosed by rigid walls, is a basement.
The term used to describe the portion of an elevated building below the lowest elevated floor that is either partially or fully shut-in by rigid walls is called a basement. This area can be found in different types of buildings, ranging from single-family homes to apartment buildings.
The basement often serves as a foundation or the lowest level of a building, from where the construction rises towards the upper floors in a systematic manner. It is distinguished from other parts of a building, such as the attic, which is typically found at the topmost part of a building, and the roof, which is the outermost covering on the top.