Siddhi is the sixteenth nityayoga, and its name means success, accomplishment, and attainment. It is one of the auspicious yogas, held highly favourable for the completion of undertakings and for any work aimed at a definite result, a name that in the wider tradition also denotes spiritual attainment. Reckoned from the summed longitude of Sun and Moon divided into twenty-seven parts, it advances as the lights draw apart. In the panchanga it lends its quality of successful attainment to the reckoning of an auspicious time. These are the conventions of the calendar tradition, in which a benefic yoga such as Siddhi is sought for endeavours meant to succeed; the astronomical basis is the combined motion of Sun and Moon.
Siddhi, accomplishment, is one of the most heavily loaded terms in the whole of Indian religious vocabulary, naming in the yogic and tantric traditions the supernatural powers, levitation, invisibility and the like, said to arise as a by-product of advanced meditative practice, and naming in ordinary Hindu devotional life the successful outcome of any undertaking, most famously invoked in the phrase Siddhi Vinayak, the epithet of Ganesha as the granter of success that gives its name to some of India's most visited temples. The nityayoga's calendrical sense, a day auspicious for bringing work to completion, is the least specialised of these several uses but draws on the same underlying root sense of a goal successfully reached.