The Moon (in Greek: σελήνη Selene, in Latin: Luna) is Earth's only natural satellite. It is one of the largest natural satellites in the Solar System, and, among planetary satellites, the largest relative to the size of the planet it orbits (its primary). It is the second-densest satellite among those whose densities are known (after Jupiter's satellite Io).

The Moon is thought to have formed approximately 4.5 billion years ago, not long after Earth. There are several hypotheses for its origin; the most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia.

The Moon is in synchronous rotation with Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters. It is the second-brightest regularly visible celestial object in Earth's sky after the Sun, as measured by illuminance on Earth's surface. Although it can appear a very bright white, its surface is actually dark, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art, and mythology.

Moon (styled as Moon.) is a Japanese adult visual novel developed by Tactics, a brand of Nexton, released on November 21, 1997 playable on Windows PCs. The game was described by the development team as a "Reaching the Heart AVG" (心に届くAVG, Kokoro ni Todoku AVG). The story follows the protagonist Ikumi Amasawa, a girl who joins an organization called Fargo in the hopes of discovering why and how her mother died, who was a member of the same group. The gameplay in Moon follows a branching plot line which offers pre-determined scenarios with courses of interaction, and focuses on the three female main characters. The game ranked twice in the national top 50 for best-selling PC games sold in Japan.

Much of the staff that created the game later became the founding members of the visual novel brand Key. Moon was the starting point for Key's origins, and was the first time the principal Key team was formed. A novel based on the game written by Midori Tateyama was released in July 1998 by Movic. The game's original soundtrack was released bundled with Dōsei's soundtrack in August 2000 at Comiket 58; Dōsei was Tactics' first game. Moon has been referenced in other media not directly related to the game, such as in Tactics' third game One: Kagayaku Kisetsu e, and in the second anime adaptation of Key's first game Kanon.

A natural satellite is a celestial body that orbits another celestial body of greater mass (e.g., a planet, star, or dwarf planet), which is called its primary. For example, the Moon is a natural satellite of Earth, and Earth is a natural satellite of the Sun.

In the Solar System there are 173 known natural satellites which orbit within 6 planetary satellite systems. In addition, several other objects are known to have satellites, including three IAU-listed dwarf planets: Pluto, Haumea, and Eris.As of January 2012, over 200 minor-planet moons have been discovered. There are 76 known objects in the asteroid belt with satellites (five with two each), four Jupiter trojans, 39 near-Earth objects (two with two satellites each), and 14 Mars-crossers. There are also 84 known natural satellites of trans-Neptunian objects. Some 150 additional small bodies have been observed within the rings of Saturn, but only a few were tracked long enough to establish orbits. Planets around other stars are likely to have satellites as well, and although numerous candidates have been detected to date, none have yet been confirmed.