The term "open world" is used to describe an inhabitable planet on the mortal plane that lacks a barrier.
Without a barrier there is nothing separating the world from the plane of Heaven and Hell. Mortals on the world can freely interact with any angels, demons, or other creatures that might find their way there via interdimensional gates or teleportation.
Open worlds are often a result of dimensional rifts breaking down what was once a regular mortal world. Some open worlds have never had a barrier because they have no sentient life that has been deemed particularly important to protect from Hell. Others because they are a direct extension of Heaven's territory, used for colonization or the like.
See the Open Worlds category for examples of open worlds.