Sector 45 was a region of space. A portion of the Romulan Neutral Zone was located in this sector. In 2364, the Federation starships USS Tranquility Base, USS Non Sequitur, and USS Max Plank were ordered by Starfleet Command to increase their sensor drone surveillance in this sector. (TNG: "Conspiracy")
The sector was never mentioned in dialog; it was mentioned on mission orders.