Magical doorways between realms
Portals are the doorways to and from Sigil. A portal can connect to any layer of any plane at any point – it’s just that one end is always anchored in Sigil. Step through an archway in Sigil and a sod might find himself on the 447th layer of the Abyss or the sixth level of Mount Celestia.
Portals don’t pass through any other planes. It’s not like a conduit, where the traveler still has to go through the Astral Plane (although in just seconds). Portals directly link two places.
Portals aren’t as hostile as vortices. A cutter doesn’t have to figure out how to reach the center of a volcano or the bottom of an ocean in order to use a portal. Most of them are easy to reach and pass through — provided a body knows where to look.
A portal may be easy to reach, but to do that a sod’s got to find it, and that’s another matter entirely. Portals generally don’t advertise themselves. They don’t glow with strange colors, and a being can’t look through one and see the destination on the other side. They don’t detect as magical, but their presence can be discovered with a true seeing spell, though even this won’t reveal where they go. A basher can walk through a portal and have nothing at all happen, too, because each one takes a special gate key.