High Quality Content by WIKIPEDIA articles! A homoeoid is a shell bounded by two concentric, similar ellipses (2D) or ellipsoids(3D). When the thickness of the shell becomes negligible, it is called a thin homoeoid.A homoeoid can be used as a construction element of a matter or charge distribution. The gravitational or electromagnetic potential of a homoeoid homogeneously filled with matter or charge is constant inside the shell. This means that a test mass or charge will not feel any force inside the shell.