Multiple realities(covers information from several alternate timelines)
Memory Delta was a planetoid.
In 2367, the location of Memory Delta was labeled on a tactical situation monitor in Captain Benjamin Maxwell's ready room on the USS Phoenix. (TNG: "The Wounded", okudagram)
In 2366, in an alternate timeline, during the Federation-Klingon War, the location of Memory Delta was labeled on a tactical situation monitor in the ready room aboard the USS Enterprise-D. (TNG: "Yesterday's Enterprise", okudagram)
This planetoid was only mentioned in writing.