Eridani D was a planet in the Eridani system with at least seven moons.
On Michael Burnham's tenth birthday, in 2236, her foster mother Amanda Grayson traveled with her on a shuttle to this planet's system. (DIS: "Lethe")
"Eridani" generally refers to stars in the constellation Eridanus, which is also home to 40 Eridani A, the location of Vulcan.
The location of this planet is inferred from the position of its star system in charts seen in several Star Trek: Strange New Worlds episodes.