basement

noun

1. a story of a building, partly or wholly underground.
2. (in classical and Renaissance architecture) the portion of a building beneath the principal story, treated as a single compositional unit.
3. the lowermost portion of a structure.
4. the substructure of a columnar or arched construction.

noun

1. 1. a partly or wholly underground storey of a building, esp the one immediately below the main floorCompare cellar
   2. (as modifier)a basement flat
2. the foundation or substructure of a wall or building
3. geology a part of the earth’s crust formed of hard igneous or metamorphic rock that lies beneath the cover of soft sedimentary rock, sediment, and soil

n.

“lowest story of a building except the cellar,” 1730, from base (n.) + -ment.