How to name pocket dimensions in fantasy and sci-fi
A pocket dimension is a closed, self-contained space accessible only from a specific point in the main world. Doctor Who has the TARDIS; Adventure Time has the Croak Dungeon; Stranger Things has the Upside Down. The name should suggest three things: containment (it's a closed place), strangeness (it's not a normal place) and portal (something lets you in).
The most-used resource is architectural metaphor: 'pocket', 'anteroom', 'vestibule', 'recess'. It works because the reader already has those mental shapes. Variants like 'fold' or 'interstice' add more abstract texture. If your world has formalized magic (guilds, schools), dimensions can have technical names: The Deanery, The Third Vault, Royal Subknot.
Consider atmosphere. A friendly comforting dimension (refuge for protagonists) calls for soft name: The Slow Tea Room. A hostile dimension calls for oppressive name: The Locked Antechamber. The name's sonic texture communicates before its literal meaning. Practice reading the name aloud: does it sound like refuge, trap, or mystery?
Pocket dimensions in different genres
In classic fantasy, pocket dimensions are magic items: D&D's Bag of Holding, the Undefined Space of Wizards. Their names are functional and descriptive. In urban fantasy (The Magicians, Neverwhere), they often have parallel geographic names to the real world: 'London Below', 'Brakebills South'.
In sci-fi, pocket dimensions are called 'subspaces', 'quantum enclaves', 'Planckian folds'. Star Trek uses compound technical names; The Expanse uses politically functional names. If your work is hard sci-fi, names can be simplified equations: Calabi-Yau Space Three, Schwarzschild Pocket.
In cosmic horror (Lovecraft, Ligotti), dimensions have inhuman names: R'lyeh, Yuggoth, the Nameless City. The aesthetic is uncomfortable pronunciation and stacked consonants. Ligotti's The Outsider calls his spaces names that look like typos. For modern horror like House of Leaves, names are architectural but impossible: 'the five and a half hallway'.
Common mistakes naming extradimensional spaces
First: generic name that doesn't differentiate. 'The Other Dimension' serves once in a novel; used three times it goes invisible. Each dimension needs its own sonic identity. If your universe has several, ensure they're distinguishable by ear: The Soft Fold and The Frozen Vault are distinct; Dimension A and Dimension B are weak.
Second: over-explaining the name within the text. If your character says 'we went to the Soft Fold, called that because the walls are velvet fabric', you killed the magic of the name. Better to show atmosphere and let the name vibrate. If the reader sees velvet walls and hears 'the Soft Fold', they connect on their own.
Third: using untranslated names without localizing. If your source is in another language, consider whether the name would benefit from your local words. Fourth: unpronounceable names without reason. If your reader stumbles three times reading the name, you lose immersion. Maintain difficulty only when it reinforces the effect (cosmic horror justifies it).
The name as narrative door
A pocket dimension's name can be the first clue of the narrative mystery. If your novel introduces the Antechamber of the Absent Potter, the reader is left with questions: who is the potter?, why are they absent?, what did they do there? Planting clues in the name saves you later exposition. Susanna Clarke's Piranesi does this masterfully: the room names are puzzles to solve.
Consider tiered nomenclature. If your world has magical hierarchy, dimensions can have official names (long, ceremonious) and popular names (short, affectionate). 'The Third Chamber of the Vibrating Council' might be called 'The Green Cube' by inhabitants. This tension between registers enriches worldbuilding.
The name can mutate throughout the story. A dimension called The Refuge in chapter one can appear as The Soft Trap in the last, without changing the place but changing the protagonist's perspective. The Magicians does this with Fillory: it begins as enchanted country and ends as something else. Use renaming as a tool for emotional arc.