A remote planet named as the origin of the Traveler, the being whose thought-driven mastery of space and time carried the Enterprise-D far beyond the galaxy.
Tau Alpha C is a very distant world, mentioned only in dialogue. In the Next Generation episode Where No One Has Gone Before (2364), the propulsion specialist Kosinski comes aboard with an unassuming assistant said to be from Tau Alpha C. The assistant, later known as the Traveler, is the real source of Kosinski's results and accidentally hurls the ship to another galaxy and beyond.
In Remember Me, Wesley Crusher tries to reach the planet for help when his mother is trapped in a collapsing warp bubble. The Traveler arrives in person instead.
The Traveler returns in Journey's End to guide Wesley towards a new plane of existence. Star Trek: Picard later shows Wesley as one of the Travelers himself.