frustum
Concise Oxford English Dictionary © 2008 Oxford University Press:
frustum /ˈfrʌstəm/
▶noun (pl. frusta /-tə/ or frustums) Geometry the portion of a cone or pyramid which remains after its upper part has been cut off by a plane parallel to its base, or which is intercepted between two such planes.
– origin C17: from L., ‘piece cut off’.
'frustum' also found in these Oxford entries:

