Miridian VI was the unhabited sixth planet orbiting the primary Miridian. This planet was located on the edge of the Romulan Neutral Zone, in the Beta Quadrant.

When Barash posed as the Human boy Ethan on Alpha Onias III in 2367, he fabricated a story about his family coming to Miridan VI after a research station had been set up on the planet in 2366. After arriving at the station in 2367, the Romulans attacked the station and captured Ethan and his family. Commander Riker felt it was odd that the USS Enterprise-D hadn't been advised by Starfleet Command about this attack. (TNG: "Future Imperfect"; DIS: "Magic to Make the Sanest Man Go Mad")

In 2371, the location of Miridian VI was labeled in the star chart Data and Picard were studying in stellar cartography aboard the USS Enterprise-D. (Star Trek Generations, okudagram)

"MER-id-dee-un" was the pronunciation for Miridian from the script pronunciation guide. [1]
Miridian, the primary of Miridian VI, was located in the Beta Quadrant on a star chart seen in the Star Trek: Discovery episode "Magic to Make the Sanest Man Go Mad".