A legacy civilization described a culture that Starfleet had made first contact with long ago, but did not maintain frequent contact with. In 2380, Captain Carol Freeman described the Betans as a legacy civilization.
While visiting Beta III, Freeman voiced frustration that Starfleet had not checked on the Betans enough since Captain James T. Kirk defeated Landru in 2267, as they were considered a "known culture." (LD: "No Small Parts")