Moitié is a French word derived from the Latin medietas, meaning "medium measure", "mean value" or "half". Its general use in traditional astrological texts is either to define a half-measure of any planet's orb (thereby identifying the distance that the orb extends on either side of the planet), or to denote the mean value obtained when two planetary orbs are added together and then halved to find the distance at which their rays unite.

To understand how the meaning of the word can alter according to context, it helps to realise that older authors usually expected a planet's influence to become noticeable on another planet whenever its conjunction or aspect with the other planet fell within the range of its own planetary orb. A standard set of planetary orbs had become widely reported by the 8th century (see table below), with most authors reporting that the Sun's orb extends over 30° of the zodiac, 15° either side of the centre of its body. This 15° limit was therefore often described as the moiety of the Sun's orb, being only half of its full radius; and any planet within 15° of a conjunction with the Sun is said to enter its beams (or go under its rays) regardless of the length of its own planetary orb. The same approach applied to the other planets.