A Wormhole is a theoretical shortcut through spacetime that connects two distant locations.
Wormholes feature prominently in science fiction, but at the moment are just that, fiction. Wormholes have not been proven to exist, nor have they been ruled out. They are currently allowed within the structure of known physics. Whether or not wormholes can exist is dependent on the exact nature of spacetime which is not completely known to science.
Space and time are often thought of separately, but in reality they are deeply connected. It seems odd to us to view three dimensions of space separately, but time is just as connected as the three spatial dimensions. In fact there are more dimensions - the exact number of which is being hotly debated. An exact place or location in space is called a point, in spacetime it is called an event. An easy analogy is a birthday party. It has a location, plus a time that it starts. It's an event. Show up too early or too late and you've missed out on the fun (and the scary balloon animal clown).
Spacetime can be thought of as a surface. Think of the surface of an object like a jelly donut. An ant on the surface of the donut would take awhile to walk from one side of the donut to the other. Instead, the ant could eat through the donut and get to the other side faster (assuming it doesn't get stuck in the jelly). In spacetime, it is theoretically possible to burrow through from one surface in one location or event to another surface in another location or event. The intriguing part of this is that it literally bypasses the barrier of light speed. It is possible to travel between locations that are separated by enormous distances - essentially any distance at all.
There are several types of proposed wormholes. An intra-universe wormhole connects two events or locations in the same universe. It is completely contained within the universe. An inter-universe wormhole connects two events or locations in parallel universes. This would get really interesting since parallel universes might have different laws of physics. Wormholes that connect different events specifically are called timeholes. Lorentzian wormholes are a theoretical proposal emerging from the study of gravity. Euclidean wormholes are theoretical proposal emerging from the field of particle physics. A special type of Lorentzian wormhole that allows for actual travel by people or other objects without being destroyed in the process is called a traversable wormhole. This is the type of wormhole that we are most familiar with from science fiction shows like Star Trek Deep Space Nine or Stargate SG-1.
If and when we develop and prove a viable theory of quantum gravity, we may finally be able to determine if wormholes exist. This may happen quite soon, as the LHC (Large Hadron Collider) at CERN goes online within the next year, and may provide the information scientists need develop a theory of quantum gravity.
