Wormholes are naturally occurring portals that connect two locations or time periods across vast distances. They may also connect different dimensions. Wormholes may be stable or may be unstable and prone to collapse. People, creatures, or objects trapped within a collapsing or collapsed fictional wormhole may undergo deadly or bizarre changes, effects, or events. Future technologies or fantastic abilities may help to stabilize fictional wormholes.

Worldbuilders using wormholes should consider how common they are (there may be areas of space without wormholes and other areas riddled with them). If rare, these sites are likely of great strategic importance and may be sites of great conflict. During peacetime they may be hubs of trade or a portion of a trade route. If wormholes can be generated at will or by design they may function more similarly to teleportation gates. Worldbuilders should consider what areas of space a wormhole may connect and what civilizations or creatures they may bring into contact.

Wormholes are most often depicted as existing in space but could also be created on a planet.

Fictional wormholes could be a cavernous maw.

While wormholes can hypothetically exist given our current understanding of the general theory of relativity, none have so far been observed. Hypothetical primordial wormholes would be microscopic in size, though theoretically could have expanded along with the growing universe. Wormholes are predicted to be very unstable, though there are some theories that they could be stabilized using exotic matter. As far as what they connect, most scientists believe they would link different regions of space, some think they could link different universes, and some (though this is argued) believe they may be used to travel through time.

Wormholes may also be referred to as Einstein-Rosen bridges, after Dr. Albert Einstein and Dr. Nathan Rosen.