basement

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The substructure or foundation of a building.
2. n. The lowest habitable story of a building, usually below ground level.
3. n. Geology A complex of undifferentiated igneous and metamorphic rocks underlying sedimentary strata.
4. n. Slang The last place or lowest level, as in competitive standings.
5. n. Chiefly New England A public toilet, especially one in a school.

Century Dictionary and Cyclopedia

1. n. The lower or fundamental portion; a base.
2. n. In architecture: The portion of the elevation of a structure which performs the function in the design of constituting a support to those portions which come above it; especially, the substructure of a columnar or arched construction, but also the lowest member in the design of a wall, etc. Compare base, 3.
3. n. A floor or story which is wholly or in part beneath the surface of the ground, but is usually, as distinguished from a cellar, well lighted, and fitted up and used for household or other usual purposes.
4. n. The act of basing, or the state of being based.

Wiktionary

1. n. A floor of a building below ground level.
2. n. sports, informal Last place in a sports conference standings.

GNU Webster's 1913

1. n. (Arch.) 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.

WordNet 3.0

1. n. the lowermost portion of a structure partly or wholly below ground level; often used for storage
2. n. the ground floor facade or interior in Renaissance architecture