Aishwarya or siddhi is derived from the word Ishwara (the supreme person). It denotes superhuman power or the faculty of eight kinds possessed by Ishwara (God) – anima (minuteness), laghima (lightness), praptih (the power to obtain anything), prakamyam (strong willpower), mahima (the power of increasing one’s size), isitvam (greatness or superiority), vasitvam (power to attract or subdue) and kamavasayita (suppression of desire).
Anu (the atom) is minute. The power of minuteness enables one to reduce oneself to a very small size and move about without being seen by anyone. The power of lightness, light as air, enables one to rise up in the sky. The ability to attain the desired object is the next superhuman faculty. A person with a strong will-power is able to accomplish one’s desires. The greatness of Mahadeva Shiva lies in his retaining the fourteen worlds in his bosom. This power makes all the animate beings abide by the command of gods (isitva). The mortals are bound by the results of their actions in their past birth carried on to their next birth. But the possessor of the faculty to subdue (vasitva) is able to overcome all the past impressions arising from nescience and is able to be independent.
The Puranas and the treatises on yoga explain the term siddhi as an impediment to the achievement of the goal of the union of the individual self with the Supreme Being. Linga Purana (I.9) states that, which one practices Yoga with its eight constituents – yama (control of senses), niyama (control of the mental states), asana (pose), pranayama (control of breath), pratyahara (contraction), dharana (establishment), dhyana (concentration) and Samadhi (meditation) – there arise many obstacles which the earnest practitioner of yoga is able to get over. But other impediments arise in the form of six siddhis, which one has to avoid in their initial stages.
Besides these, Linga Purana (I.9) describes the yogic powers related to each element. In relation to the earth – bulkiness, leanness, regaining infancy, youthfulness, decrepitude, assuming different shapes, holding the body without an earth element, etc. In relation to water – to stay under water for a long time and to come out at one’s will, to drink up the ocean, to let the water spring up wherever one wills, to convert anything into a tasty substance, and to keep the body free from wounds. In relation to fire – to create fire from the body, to remain free from fear of being scorched by fire, to keep something unburnt even when the whole world is burning, to hold fire in the water or in the palms, to produce fire by mere remembrance, to recreate something which has been reduced to ashes and to possess a body with two elements – air and ether. In relation to air – to move as swiftly as the mind, to enter the bodies of living beings, to hold a heavy weight, to achieve lightness and heaviness, to hold the air with the palms, to shake the earth with the tip of the finger, and to create bodies out air.