A subspace rupture is an anomaly in subspace.
In the mid-23rd century, a subspace rupture was discovered in the Hanoli star system. A Vulcan starship crew investigated it and detonated a pulse wave torpedo in the tear. This caused the rupture to expand, destroying the Vulcans and the entire Hanoli system. The rupture reached critical mass and disappeared.
In 2369, Jadzia Dax, researching a rupture near Deep Space 9, read about the Hanoli subspace rupture. A group of aliens studying humanoid imaginations made the subspace rupture real, which the crew tried in vain to study before it "destroyed" the station, before Benjamin Sisko realised it too was part of their imaginations. (DS9 episode: "If Wishes Were Horses")