Mithuna is the third rashi, the Vedic sign of the twins or the couple, corresponding to Gemini. In jyotisha it is ruled by Mercury (Budha). Its quality is dual or mutable (dvisvabhava), and its element is air (vayu tattva). Jyotisha reads Mithuna through Mercury's versatility: communication, curiosity, exchange, and the two-sided, adaptable nature the twins symbolise. As a sidereal sign it is fixed to the stars and sits roughly 24 degrees behind the tropical Gemini. Mercury's rulership of the twins is common to Western and Vedic astrology alike. Jyotisha then reads a planet placed here through the additional machinery the tradition supplies, the nakshatra it falls in, the divisional charts, and the planetary period (dasha) then running, so the sign is only the first layer of a much denser interpretive system.
Mithuna translates as a pair or a couple rather than specifically twins, a subtly different sense from the Greek didymoi it renders, which does mean twins outright, and the Sanskrit name reflects the general fact of pairing more than the specific mythological brothers Castor and Pollux the Greek sign evokes. That transmission runs through the Yavanajataka, a Sanskrit versification of a Greek astrological text of roughly the second through fourth centuries CE, though the twelve equal signs themselves originate earlier still, in Babylonian celestial divination of roughly the fifth or fourth century BCE, with no twelve-sign zodiac of any kind attested in the older Vedic corpus. Mercury's rulership of Mithuna, shared with the tropical Gemini, belongs to a later classical layer, standardly credited to compilations such as the Brihat Parashara Hora Shastra of the seventh or eighth century CE.
Sources Brihat Parashara Hora ShastraSage Parashara (attributed) with The Yavanajataka of SphujidhvajaDavid Pingree, Jyotihsastra: Astral and Mathematical LiteratureDavid Pingree