A "pocket dimension" refers to a small, separate space or alternate reality that can exist within a larger one. It is like a hidden, magical pocket or room that exists outside of our usual understanding of space and time. Full definition
In Fire Emblem Fates the incorporation of this mechanic has a somewhat flimsy connection to the plot, as it is never really explained how it is possible for the Fire Emblem Fates children to fight alongside their parents, aside from the fact that the protagonists hide their children in pocket dimensions in order to protect them. (igcritic.com)
The new engine's vest - pocket dimensions permit manufacturers to bolt on all sorts of plumbing to purify the exhaust while keeping the basic unit small, light, uncomplicated and reasonably priced. (si.com)
Writer Jon Spaihts has worked on his own draft of the script, which may include some notable appearances from the magic - based heroes at their usual stomping ground - Eddie Deacon's pocket dimension bar. (screenrant.com)