A quantum singularity is a region of infinite density or "infinite curvature", thus exerting extremely strong gravitational forces. A quantum singularity is formed when a star collapses in on itself and is also commonly known as a black hole.
The Borg were able to access Fluidic space via a quantum singularity. Species 8472 used the Borg's methods of creating quantum singularities and began an invasion of our realm. On the USS Voyager, Seven of Nine accessed the main deflector and created a singularity to send Voyager to fluidic space. (ST: Voyager: "Scorpion")