Homoeoid and focaloid

A homoeoid or homeoid 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. The name homoeoid was coined by Lord Kelvin and Peter Tait. Closely related is the focaloid, a shell between concentric, confocal ellipses or ellipsoids.

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