basement

[beys-muh nt] /ˈbeɪs mənt/
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.
Origin
1720-30; base1 + -ment
British Dictionary definitions for basement

basement

/ˈbeɪsmənt/
noun
1.
  1. a partly or wholly underground storey of a building, esp the one immediately below the main floor Compare 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
Word Origin and History for basement
n.

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