Artist's depiction of a Wormhole.
Wormholes are a type of spatial phenomena in which a Singularity such as a Black Hole links up with another black hole somewhere else in the universe, presumably through extra dimensions, creating a shortcut through space and time. Wormholes are frequently used as a means of Faster than light travel and occasionally as Time Travel.
Wormholes have two characteristics: natural or created; two types: stable or unstable; and two durations: temporary or permanent. Natural wormholes are created by the usual processes of the universe. Created (or artificial) wormholes are created by some entity, either directly or through use of a device. Stable wormholes are ones that remain open for specific uses or period of time. Unstable wormholes collapse immediately after formation, or the wormhole's open time is erratic. Temporary wormholes are those that appear randomly, once, or reappear in different places on an irregular basis. Permanent wormholes are those that reoccur at the same location, although the destination need not be the same every time.
Stable wormholes are rare in nature, though sometimes incredibly advanced civilizations have been known to create them. Unstable wormholes are moving rapidly through spacetime, on both ends. This often leads to hazards to travelers moving through the wormhole.
Permanent wormholes are almost always created (implicitly in the case of Star Trek: Deep Space Nine, as explained below; or explicitly, as in the case of the Stargete franchise.)
The Bajoran Wormhole in the Bajor System of the Alpha Quadrant in the Milky Way was one example of a rare stable wormhole. It connected the Alpha Quadrant to the Gamma Quadrant. Additionally, a sentient species lived in the wormhole and may be responsible for the wormhole's stability. (Star Trek franchise: "Star Trek: Deep Space Nine")
The Stargates create stable wormholes on a regular basis from one gate to another. Matter can move through these wormholes to be compressed into a matter stream and later reassembled. The Ancients were responsible for inventing the Stargates. (Stargate franchise: "Stargate SG-1")
A time machine that can transport people to a distant place may be considered to be a wormhole. This may be a judgment call by the reader, if the machine's technology is not described or is not explicitly stated as either using a wormhole or is not using one.
