Black star
From Memory Alpha, the free Star Trek reference.
Black star was a type of star that had high gravitational attraction. They could remain undetected until encountered by entering their gravity field.
In 2267, the USS Enterprise, en route to Starbase 9 for resupply, was dragged towards the gravity of a previously unknown black star in the area of the base. While using all available warp power in reverse to escape the star, the breakaway sent the Enterprise into a time warp due to the slingshot effect. The whiplash threw the starship almost three centuries backward through time and to a low atmospheric orbit around Earth, as the Sol system was located in the heading they were pulling away to. (TOS: "Tomorrow is Yesterday")
Background
Instead of a fictional type of star, this designation might also refer to a neutron star or a black hole. Both have high mass, and therefore high gravity, and are objects that do not emit visible light. Episode writer D.C. Fontana probably meant a black hole, as the concept of collapsed stellar objects that do not emit radiation dates to around 1916 and Karl Schwarzschild, but the term black hole did not enter the vernacular until 1967 – after this episode was written.
