Miridian VI is an uninhabited planet located in Federation space near the Romulan Neutral Zone.
When Barash posed as the Human boy Ethan on Alpha Onias III in 2367, he claimed to have been captured by Romulans during an attack on a Federation outpost on Miridian VI. Commander Riker was surprised to hear about this outpost as he had no knowledge about it (which ultimately turned out to be true). (TNG: "Future Imperfect")