A virtual portal is some kind of teleport to the other point of the map space. The portal enables you to view and pass through it. Imagine it like a mirror in the center of the scene, showing space that is only virtually in front of you, but you can go there. The portal could lead you in the circle while you think you are going straight forward. The virtual space of the other side of the portal could change its scale and orientation, allowing effects as in "Alice in the Wonderland". Physically it's like a space-wormhole (space-wrapping). It is simple to create a wall of portals that show a 360 degree spherical view from a specified point in the map space. So you are looking a little outside of 3D, but in 4D.