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Siddha (Tamil "great thinker/wise man"; Sanskrit, "perfected one") is a term that is used widely in Indian religions and culture. It means "one who is accomplished". It refers to perfected masters who have achieved a high degree of physical as well as spiritual perfection or enlightenment. In Jainism, the term is used to refer the liberated souls. Siddha may also refer to one who has attained a siddhi, paranormal capabilities.
Siddhas may broadly refer to siddhars, naths, ascetics, sadhus, or yogis because they all practice sādhanā.
The Svetasvatara (II.12) presupposes a Siddha body.

Source: Wikipedia