A point of collapsed matter with gravity so intense that it bends space and time, one of which trapped Voyager behind its event horizon and showed the crew a reflection of their own ship in Parallax.
A quantum singularity is Star Trek's usual term for a black hole or similar collapsed object. Its event horizon marks the boundary beyond which nothing ordinarily escapes, and the gravity near it distorts time as well as space.
In the Voyager episode Parallax in 2371, the ship answers a distress call from a vessel caught in a type 4 quantum singularity. After a failed rescue the crew realise that the other ship is Voyager itself, a time-delayed image, and that they have been inside the event horizon all along. Janeway and B'Elanna Torres take a shuttle to widen the crack through which the ship entered.
Romulan warbirds use an artificial singularity as their power source, as Timescape shows, and Voyager meets a sentient star-like predator that mimics a wormhole in Bliss. The 2009 film Star Trek creates black holes with red matter.