Where the space between worlds is thin are places in fictional settings where two or more worlds, realms, or dimensions connect to one another or are ‘close.’ This typically means that travel between these locations is possible or that events in one location can influence the events of the other. It is at these sites that characters or creatures may cross between worlds (perhaps because of a portal or teleportation gate, maybe because of a temporary path or bridge, or maybe because they simply become lost and wander between magical places). Fictional places where the space between worlds is thin are used in fantasy, science fiction worlds, super hero worlds, and other fictional settings where more than one world, realm, or dimension exists. They often connect the mortal realm with spirit worlds, god domains, the underworld, or multiple dimensions of a multiverse.

Places where the space between worlds is thin can be anywhere, but often worldbuilders choose remote locations or areas where characters can become lost such as ley lines and magical fogs. Portals, teleportation gates, or hidden doors may be necessary to make the journey or may mark the site. Ancient ruins, altars, or other elements may also be present. The connection points may not be on level ground on both sides of the connection, however. Entering on ground level may lead to exiting in the skies, underground, or underwater.

Worlds may be thin only at certain times of the year or may open only when certain events occur (a sacrifice is made, a ritual performed, or a novel technological process is employed).