Basement

n. cellar, story of a building which is underground

A basement is one or more floors of a building that are either completely or partially below the ground floor.  Slab-on-grade buildings do not have basements. Basements are typically used as a utility space for a building where such items as the furnace, water heater, car park, and air-conditioning system are located; so also are amenities such as the electrical distribution system, and cable television distribution point.
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Noun
1. the lowermost portion of a structure partly or wholly below ground level; often used for storage
(synonym) cellar
(hypernym) floor, level, storey, story
(hyponym) cellarage
2. the ground floor facade or interior in Renaissance architecture
(hypernym) support

n. base, foundation, bottom layer, principal element, fundamental part, fortified area

(a.)
The outer wall of the ground story of a building, or of a part of that story, when treated as a distinct substructure. ( See Base, n., 3 (a).) Hence: The rooms of a ground floor, collectively.