noun
ellipsoid
A three-dimensional shape whose plane sections are ellipses or circles; used in geometry and related fields.
地球は完全な球ではなく、楕円体に近い形をしている。
The Earth is not a perfect sphere but is shaped more like an ellipsoid.
球体 is a sphere; 楕円体 is an ellipsoid, which is a more general shape that includes spheres as a special case.
Compound of 楕円 (ellipse) + 体 (body, solid). The exact historical derivation is uncertain, but the term is a straightforward technical coinage.