Siddha is the twenty-first nityayoga, and its name means accomplished, perfected, or one who has attained. It is one of the auspicious yogas, favourable for undertakings aimed at success and for spiritual practice, the word siddha in the wider tradition names an accomplished adept. Reckoned like the others from the summed longitude of Sun and Moon divided into twenty-seven parts, it advances through the month. In the panchanga it lends its quality of attainment to the reckoning of an auspicious moment, one of the five daily limbs weighed for muhurta. These are the conventions of the calendar tradition, in which each of the twenty-seven yogas carries a settled character; Siddha falls among the benefic set. The astronomical basis is the combined motion of Sun and Moon.
Siddha, an adjective built from the same root as Siddhi, the sixteenth yoga, names in the wider tradition an accomplished spiritual adept, one who has attained the supernatural powers and liberated state that practice is said to confer, and Siddha in this sense is also the name of an entire medical and alchemical tradition, Siddha medicine, practised principally in Tamil Nadu and associated with legendary adept-physicians distinct from the more widespread Ayurvedic system. The nityayoga's own use of the word is the general calendrical sense of favourable attainment rather than a direct reference to either the yogic adept or the medical tradition, though all three uses draw on the same underlying sense of a state successfully reached or perfected.