A wormhole is a "tunnel" connecting two separate points of space-time. It consists of two "holes" in space-time connected by a conduit outside normal space, through 'subspace'. Since this conduit through subspace is shorter than the distance between the two points in normal space, almost instantaneous travel is possible between the two points. If a wormhole is large enough, and stable enough, a starship (or other traveller) could enter one end and exit the other, thus shortening a journey. A wormhole can also connect two different points in time since the conduit is outside normal space-time, enabling it to return at any place, at any time.
Before knowing where the Harry Kim wormhole ends, Tuvok stated that there is a 75% chance that it doesn't lead to the Alpha Quadrant. We may conclude that wormholes never leave the Milky Way (VOY: "Eye of the Needle").
In physics, a wormhole is a hypothetical topological feature of spacetime that is essentially a 'shortcut' through space and time. A wormhole has at least two mouths which are connected to a single throat or tube. If the wormhole is traversable, matter can 'travel' from one mouth to the other by passing through the throat. While there is no observational evidence for wormholes, spacetimes containing wormholes are known to be valid solutions in general relativity.
The term wormhole was coined by the American theoretical physicist John Wheeler in 1957. However, the idea of wormholes was invented already in 1921 by the German mathematician Hermann Weyl in connection with his analysis of mass in terms of electromagnetic field energy.
The name "wormhole" comes from an analogy used to explain the phenomenon. If a worm is travelling over the skin of an apple, then the worm could take a shortcut to the opposite side of the apple's skin by burrowing through its center, rather than travelling the entire distance around, just as a wormhole traveler could take a shortcut to the opposite side of the universe through a topologically nontrivial tunnel.