A pocket universe is a type of reality that is dimensionally located within a universe.
History and specifics
A pocket universe can be accessed from certain fixed points within the universe is exists alongside, but objects within the pocket universe cannot be perceived in the universe than contains it. The D'myurj and Brethren originated in a pocket universe that could be accessed from Sector 221-G. (New Frontier)
The Bajoran wormhole was considered a pocket universe. (DS9 novel: Bloodletter)
The Inamuri entity existed in a pocket universe. (DS9 novel: Twilight)
A 100,000 year-old Loregni A.I. controlled a network of ancient defense satellites in system FGC 6619. Even in the 23rd century, they were capable of trapping intruding starships in a pocket universe. (Adventures RPG module: Mission Briefs: Ancient Civilizations mission: "Closed Borders")
76,000 years ago, after the Zyndam civil war, survivors of the Zyndam civilization exiled themselves to a pocket universe. (Adventures RPG module: Mission Briefs: Ancient Civilizations mission: "Quest for Zyndam")