This tag is to be used when considering the geometry of the universe itself. It refers not to extraterrestrial species, but to geometries completely foreign to normal experience.
A geometry different from that of normal experience would be a non-euclidean geometry. 3 dimensional non-euclidean geometries are typically extremely difficult to visualize. One helpful notion is that of a quotient, in which you take a space, and set certain points equal. This allows you to say, glue the edges of a cube, or declare that all points are 5 feet left of themselves (see 3). It is recommended that you consider 3-manifolds (or 4-manifold if considering time), as they act like normal space in arbitrarily small regions. If your geometry is not a manifold, you would need a truly alien world to support your alien geometry.