Subspace

Subspace is an integral part of the space-time continuum, distinct, yet coexistent with normal space. Subspace and normal space are confluenced together. However, in some regions an interfold layer forms between the two realms. (VOY: "Real Life") Subspace has an infinite number of domains. Geordi La Forge compared it with "...a huge honeycomb with an infinite number of cells". (TNG: "Schisms")

Tetryon particles are a form of subatomic particle which can only exist naturally in subspace. If they are released into normal space, they exhibit erratic properties. This can only occur when the fabric of subspace comes into contact with normal space in some manner.

Exactly what subspace is has never been revealed on screen, though many theories, both fan-based and scientific, have been put forward. Explaining subspace would be hard, since it is used as a solution and cause of various problems throughout the series. It is implied to be the medium through which faster-than-light travel and communication is possible, similar to hyperspace.