A subspace phenomenon that reduces matter entering it to a fraction of its size, used in One Little Ship to shrink the runabout Rubicon and its crew to a few centimetres long.
Subspace compression is the effect of a rare anomaly in which space is squeezed so that anything passing inside becomes proportionally smaller, while continuing to function as normal.
In the Deep Space Nine episode One Little Ship in 2374, the Defiant investigates such an anomaly by sending in the runabout Rubicon, with Dax, O'Brien and Bashir aboard, on the end of a tractor beam. While the runabout is shrunk, Jem'Hadar board and capture the Defiant, the tether is lost and the Rubicon is thrown clear without regaining its size.
The tiny craft flies in through a plasma vent and darts about inside the ship unnoticed. O'Brien and Bashir even leave it to work among the circuits of a console, where the air molecules are too large for them to breathe for long. Their help lets Sisko retake the ship, and the runabout is restored by re-entering the anomaly.