What is "homoeoid"

n. (context mathematics English) A boundlessly thin shell (l en constrained) by a (l en couple) of similar surfaces with nearly identical (l en orientations).

A homoeoid is a shell (a bounded region) bounded by two concentric, similar ellipses (in 2D) or ellipsoids (in 3D). When the thickness of the shell becomes negligible, it is called a thin homoeoid.