A siddha has also been defined to refer to one who has attained a siddhi. The siddhis as paranormal abilities are considered emergent abilities of an individual that is on the path to siddhahood, and do not define a siddha, who is established in the Pranav - the Aum, which is the spiritual substrate of creation. The siddhi in its pure form means "the attainment of flawless identity with Reality (Brahman); perfection of Spirit."
The concept of siddhas is a prime notion in Jainism.
In South India, a siddha refers to a being who has achieved a high degree of physical as well as spiritual perfection or enlightenment. The ultimate demonstration of this is that siddhas allegedly attained physical immortality. Thus siddha, like siddhar or cittar (indigenisation of Sanskrit terms in Tamil Nadu) refers to a person who has realised the goal of a type of sadhana and become a perfected being. In Tamil Nadu, South India, where the siddha tradition is still practiced, special individuals are recognized as and called siddhas, or siddhars or cittars, who are on the path to that assumed perfection after they have taken special secret rasayanas to perfect their bodies, in order to be able to sustain prolonged meditation along with a form of pranayama which reduces the number of breaths taken by them considerably.
Siddha medicine is a form of medical treatment of diseases using substances of all possible origins in a way that balances the possible harmful effect of each substance. This form of medicine was professed and practised by siddhars who wrote their recipes on palm-leaves for the use of future generations. Siddha medicine was developed by outstanding Dravidians (ancient Tamils), locally called Cittars. Preparations are made mainly out of the parts of the plants and trees such as leaves, bark, stem, root etc, but include also mineral and some animal substances. This form of medicine is still today well known in South India. The use of metals like gold, silver and iron powders (Sanskrit bhasma) in some preparations is a special feature of siddha medicine, which claims it can detoxify metals to enable them to be used for stubborn diseases. This claim is especially relevant in the case of mercury which is relatively often used in the system; that means medicine containing purified mercury should only be received from a highly qualified practitioner of the art.