frustrum

Noun

frustum (plural frustums or frusta)

1. A cone or pyramid whose tip has been truncated by a plane parallel to its base.
   • In a parabolic conoid this difference vaniſhes, the fruſtum being always equal to a cylinder of the ſame height upon the ſection of the conoid that biſects the altitude of the fruſtum and is parallel to its baſes.1742, Colin MacLaurin, A Treatise of Fluxions, volume 1, page 25
2. A portion of a sphere, or in general any solid, delimited by two parallel planes.
   • In some women it^([the os uteri]) is flat, in many more tuberose, and forming, as it were, a frustum of a sphere;[…].1840, James Blundell, Observations on Some of the More Important Diseases of Women, page 131