Apeirotopes
An apeirotope is, like any other polytope, an unbounded hyper-surface. The difference is that whereas a polytope's hyper-surface curls back on itself to close round a finite volume of hyperspace, an apeirotope does not curl back.
Some people regard apeirotopes as just a special kind of polytope, while others regard them as rather different things.
Read more about this topic: List Of Regular Polytopes