Pocket universe

Pocket universes are a type of very small parallel universe. They are sometimes "attached" to a larger parent universe, making them literally pockets of space, but this is not a necessary feature and the name generally refers to their small size. "Small size" can be a relative and subjective thing, however. The creation of some pocket universes might be the result of alternate timelines.

Beings like the Q and the Douwd can create illusionary worlds that can form a type of pocket universe experienced by those within the illusion. (TNG: "Q-Pid", "The Survivors")

Wesley Crusher's warp field experiment in 2367 accidentally trapped his mother in a shrinking pocket universe. Wesley rescued his mother with the assistance of The Traveler (TNG: "Remember Me")

In 2382, Darrum placed Timothy Sinclair in a pocket universe built around Sinclair's unexpressed wishes (or what Darrum believed them to be). Sinclair's will eventually "shattered" the reality. In 2384, Darrum again offered Sinclair that "perfect world," but Sinclair refused, even though he knew it would result in his death. (PDN: "More to This Life", "An Evening in Gethsemene")