Pocket Dimension 

A separate dimension where things mysteriously go when lost, they are trapped with the many pens and pencils among other things we do not speak of :)

Man 1: yo i guess my binder went to the pocket dimension cause it has disappeared.
Man 2: Aw man I'm sorry, but you won't get it back from the pocket dimension.

Pocket Dimension by Peanutbuttersquirel June 3, 2018

Pocket dimension 

An alternate reality inside our universe that can only be entered through a portal. It may have a made-up timeline.

A pocket dimension is a large contrast from a 'merged dimension' (please refer to the definition of merged dimension, on Urban Dictionary.com).

The demigod opened a portal to a pocket dimension full of mythical beasts, on the planet, Mars.

Pocket dimension by Diana Lucius De Collis November 30, 2021