A region where the fabric of space weakens and two parallel universes overlap at intervals, which swallowed the USS Defiant and left Captain Kirk drifting between dimensions in The Tholian Web.
A spatial interphase is a zone in which space itself breaks down, so that at regular intervals one universe overlaps with another. Objects caught at the moment of overlap can slip out of normal space, and the distortion also affects the humanoid nervous system, driving crews to madness and violence.
In The Original Series episode The Tholian Web in 2268, the Enterprise finds the USS Defiant adrift in such a region, her crew having killed one another. The ship fades away with Kirk still aboard in an environmental suit, and Spock must wait for the next interphase to retrieve him while the Tholians spin an energy web around the Enterprise.
Enterprise returns to the idea in In a Mirror, Darkly, which reveals that the Defiant emerged in the mirror universe of 2155, where the Tholians had deliberately opened the rift and the Terran Empire's Jonathan Archer seized the ship.