Outpost 23 was a Federation outpost along the Romulan Neutral Zone. In 2367, it was the key outpost in Starfleet's defenses along the border.

In 2366, after receiving intelligence from a Romulan defector about a possible Romulan base at Nelvana III, this outpost received a warning from Starfleet Command. (TNG: "The Defector")

In 2367, in a holoprogram created for William T. Riker by Barash on Alpha Onias III, Outpost 23 was supposedly no longer of strategic importance and was slated to be the site of final peace negotiations between the Federation and the Romulan Star Empire, an apparent ruse to reveal the location of the outpost. (TNG: "Future Imperfect")

This outpost was only mentioned in dialogue.
According to Star Trek: Star Charts (p. 66), Earth Outpost 23 was located in the Beta Quadrant.