The Deneb Sector was a region in Confederation space. Proxima Centauri (now part of Sol Sector) was located in Deneb. In the event of a Kilrathi victory in the Vega Sector, the Confederation's plan was to fall back to Proxima Cenaturi and use it as a central defense location for Deneb.
Behind the scenes
The Deneb sector is mentioned in the first Wing Commander game. The sector, however, does not appear on the starmap featured with Wing Commander Prophecy instead having Proxima Centauri being part of the "Terra Quadrant", Sol Sector.