The Sanskrit word Siddhi is the shining manifestation of success emanating from the Cosmic Spirit. In mundane terms the word relates to accomplishment, perfection, or occult power.
Within the International Nath Order, Siddhi is one of the three stated aims of the Order: spiritual development toward awakening, development of occult attainment (Siddhi), and a more enlightened social order. It is a focus for serious aspirants, not a collection of tricks. Sri Kapilnath writes that such powers appear in course, as a deeper connection between the individual spirit and the Cosmic Spirit; used with care they can serve the work, but they also pull toward the world of people, things, and ideas. Siddhi is obtained by yoga with the Cosmic Spirit. The person who has gone far enough to hold these powers is a Siddha; in INO usage an Adept is one who has attained to that sphere. Do not read Siddhi as the name of the practitioner.
In Indian mythology, Siddhi is one of the two spouses of the elephant-headed divinity, Ganesha.