Earth's only natural satellite, settled by humans in the twenty-first century and home to communities such as Tycho City and New Berlin and to the Orpheus mining colony.
Luna is the formal name for Earth's Moon. In Star Trek it is the first world beyond Earth that humans settle, and by the twenty-fourth century it has tens of millions of inhabitants, as Riker tells Zefram Cochrane in Star Trek: First Contact while naming Tycho City, New Berlin and Lake Armstrong.
The Moon is seen most closely in Enterprise. In Demons and Terra Prime, set in 2155, John Frederick Paxton runs his Terra Prime movement from the Orpheus mining colony on the lunar surface, and later uses the verteron array on Mars to blast a crater in the Moon as a show of force.
Elsewhere it is mostly a familiar piece of background: a place characters come from, a landmark seen from Earth orbit, and a reminder of how ordinary life away from Earth has become.