A portal (sometimes capitalized, as "Portal") is a weakness in the fabric between dimensions — that is, a point of Inter-Dimensional Fatigue. All known portals are circular (though those generated by gravity wells might actually be spherical).
Known instances of portals are the portals generated by hyperdrive, the portal generated by the QuasiSpace Portal Spawner, the unidirectional portals from HyperSpace to TrueSpace formed by the gravity wells of TrueSpace stars, the fifteen unidirectional portals that lead from QuasiSpace to HyperSpace, and the bidirectional portal to QuasiSpace (at HyperSpace coordinates 043.8 : 637.2). Since the Arilou Lalee'lay homeworld, Falayalaralfali, is suspended in a bubble of TrueSpace within QuasiSpace, it is possible that its intrusion into QuasiSpace is also a portal. The Orz dialogue also contains a possible reference to portals ("Then the Androsynth made some *slippery places* and then Orz can *smell* [the Humans' *level*].").