A system to which the Enterprise-D was travelling in 2367 when it was caught up in a cluster of two-dimensional lifeforms and dragged towards a cosmic string.
T'lli Beta is a destination named in the Next Generation episode The Loss.
The Enterprise-D is under way to T'lli Beta when sensors show phantom readings and the ship finds that it cannot change course. It has been swept along by a shoal of creatures that exist in only two dimensions, and at the same moment Deanna Troi loses her empathic sense.
The beings are heading for a cosmic string that would tear the ship apart. Troi, working without her abilities, reasons that they are drawn to it as moths are to a flame, and Data creates a decoy that lets the ship break free. The Enterprise then resumes its journey.