Magic portal

A portal is a type of trap that takes you to a different section of the dungeon. Portals can be found:

• In a vault between dungeon level 11 and Medusa, leading to Fort Ludios.
• In the entrance to the Wizard's Tower in Gehennom.
• In a normal dungeon room, leading to the Quest.
• Somewhere on each of the Elemental Planes leading to the next.

Single-use portals are also technically generated when the Eye of the Aethiopica is used to gate between dungeon branches, although you never get to see them.

Encyclopedia entry

Portals can be Mirrors, Pictures, Standing Stones, Stone
Circles, Windows, and special gates set up for the purpose.
You will travel through them both to distant parts of the
continent and to and from our own world.  The precise manner
of their working is a Management secret.
[ The Tough Guide to Fantasyland, by Diana Wynne Jones ]