The Star Trek Expanded Universe Database is for fanon and related content. See Memory-Alpha.org for the canon Star Trek wiki.

Transphasic nebula was a type of nebula which had unique transphasic qualities which created minor disturbances in subspace.

In 2377, the Borg Collective managed to create a transwarp gate between a transphasic nebula in the Delta Quadrant and a similar nebula in the Regula Badlands region of the Beta Quadrant. The creation of these corridors allowed the Borg to attack and assimilate several Federation and Romulan colonies without encountering significant resistance.

This same conduit was later utilized by Starfleet to dispatch a fleet of starships, led by the USS Enterprise-E, into the Delta Quadrant to launch an attack on the Collective. (TNG computer game: Armada II)

The USS Daedalus encountered a similar nebula located in the Scorpius Reach in early 2378 and began an survey of the nebula. Unfortunately, the transphasic qualities of the nebula reacted badly with the Doctor's TARDIS and caused temportal instability aboard the Daedalus which came close to destroying the ship. Thankfully, the Doctor was able to work with the Daedalus crew and was able to shield the starship from the temporal instability by expanding the TARDIS's temporal shielding around her. (Star Trek: Daedalus: "Turbulence")