(covers information from several alternate timelines)
Tau Alpha C was the reported home of the The Traveler. The Enterprise computer was familiar with its inhabitants.
Considered to be very distant by Starfleet, at warp 9.5, it would take 123 days to travel from Starbase 133 to Tau Alpha C. A subspace message would take days to travel back and forth between both locations. This location was listed on Federation star charts. (TNG: "Where No One Has Gone Before", "Remember Me")
In an alternate universe created after Doctor Beverly Crusher became trapped within a warp bubble, Wesley tried to contact Tau Alpha C for assistance when individuals began disappearing from the Enterprise, with only Dr. Crusher noticing. Ultimately, that entire universe, including its Tau Alpha C, disappeared. (TNG: "Remember Me")
Background information Edit
This location was only mentioned in dialogue.
According to StarTrek.com, Tau Alpha C was the name of a star system.  Under naming conventions for stars (ex. Alpha Centauri C), the name would suggest Tau Alpha C was the third star in a trinary star system.
According to the Star Trek Encyclopedia (3rd ed., p. 499), Tau Alpha C was a "very distant planet".