Realmspace

Realmspace is a general term for anything and everything that could reasonably be considered the Material Plane, and even goes so far as to include both the Feywilds and the Shadowfell in that umbrella. These are the systems our characters typically live in, and also the systems largely under threat of invasion from dark powers like Shuriig. 
This might get confusing for some, but realmspace is not a contiguous area connecting one domain to another; it instead refers to the pockets of space inside a given "crystal sphere", the incomprehensibly large structures that contain a given solar system and protect it from the chaotic, destructive flows of the phlogiston. While realmspace itself is still considered a part of Wildspace, it is still a part of the material plane; creatures within a realmspace system still age normally, must eat and drink if they need to do so, and if they need to breathe, they have to find a way to keep doing that too (like staying aboard a spelljammer with a fresh air envelope, wearing a fish suit, etc).  
A given realmspace system or "sphere" typically has a single star (though some exceptionally rare circumstances have produced spheres with two- or even three-star systems), and usually somewhere between one and ten planets. While not unhead of, sphere's are not especially known to have more than one or two planets capable of supporting Naturalborn races and life in general.

To travel to other realmspace systems without needing to leave a sphere and traverse Wildspace, you'll need to do so via Corridor or some method of teleportation. Remember that Corridors can only deliver you to established locations (places documented here in the Atlas), and teleportation typically requires you to know where you're going or have something that came from your destination. 

Type

Plane of Existence