Siddha

Siddha 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 perfection of the intellect as well as liberation or enlightenment. In Jainism, the term is used to refer to 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.

Jainism

In Jainism, the term siddha refers to liberated souls who have destroyed all karmas and obtained moksha. They are free from the transmigratory cycle of birth and death and are above Arihantas. Siddhas do not have a body; they are soul in its purest form. They reside in the Siddhashila, which is situated at the top of the Universe. They are formless and have no passions and therefore are free from all temptations. They do not have any karmas and they do not collect new karmas.
The Śvetāmbara Ācārāṅga Sūtra 1.197 describes siddhas in this way:
According to Jains, siddhas have eight specific characteristics or qualities. The ancient Tamil Jain Classic 'Choodamani Nigandu' describes the eight characteristics in a poem, which is given below.
The following table summarizes the eight supreme qualities of a liberated soul.

Quality	Meaning	Manifestation
Kśāyika samyaktva	infinite faith or belief in the tattvas or essential principles of reality	manifested on the destruction of the faith-deluding karma
Kevala Jnāna	infinite knowledge	on the destruction of the knowledge-obscuring karma.
Kevaladarśana	infinite perception	on the destruction of the perception-obscuring karma
Anantavīrya	infinite power	on the destruction of the obstructive karma
Sūksmatva	fineness	manifested on the destruction of the life- determining karma
Avagāhan	inter-penetrability	manifested on the destruction of the name-determining karma
Agurulaghutva	literally, neither heavy nor light	manifested on the destruction of the status-determining karma
Avyābādha	undisturbed, infinite bliss	manifested on the destruction of the feeling-producing karma

Because of the quality of Sūksmatva, the liberated soul is beyond sense-perception and its knowledge of the substances is direct, without the use of the senses and the mind. The quality of avagāhan means that the liberated soul does not hinder the existence of other such souls in the same space.
A soul, after attaining siddhahood, goes to the top of the loka (as per Jain cosmology) and stays there for eternity.

Hinduism

In Hinduism, the first usage of the term siddha occurs in the Maitreya Upanishad in chapter Adhya III where the writer of the section declares "I am Siddha."

Siddhashrama

In Hindu theology, Siddhashrama is a secret land deep in the Himalayas, where great yogis, sadhus and sages who are siddhas live. The concept is similar to Tibetan mystical land of Shambhala.
Siddhashrama is referred in many Indian epics and Puranas including Ramayana and Mahabharata. In Valmiki's Ramayana it is said that Viswamitra had his hermitage in Siddhashrama, the erstwhile hermitage of Vishnu, when he appeared as the Vamana avatar. He takes Rama and Lakshmana to Siddhashrama to exterminate the rakshasas who are disturbing his religious sacrifices.

Nath sampradaya

Whenever siddha is mentioned, the 84 siddhas and 9 nathas are remembered, and it is this tradition of siddha which is known as the Nath tradition. Siddha is a term used for both mahasiddhas and Naths So a siddha may mean a siddha, a mahasiddha or a nath. The three words are used interchangeably.

In the Varna(na)ratnakara

A list of eighty-four siddhas is found in a manuscript dated Lakshmana Samvat 388 of a medieval Maithili work, the Varna Ratnakara written by Jyotirishwar Thakur, the court poet of King Harisimhadeva of Mithila. An interesting feature of this list is that the names of the most revered naths are incorporated in this list along with Buddhist siddhācāryas. The names of the siddhas found in this list are:

In the ''Hatha Yoga Pradipika''

In the first upadeśa of the Hatha Yoga Pradipika, a 15th-century text, a list of yogis is found, who are described as the Mahasiddhas. This list has a number of names common with those found in the list of the Varnaratnākara:

Tamil tradition

In Tamil Nadu, 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, 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 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 considerably reduces the number of breaths they take. Siddha were said to have special powers including flight. These eight powers are collectively known as attamasiddhigal. In Hindu cosmology, Siddhaloka is a subtle world where perfected beings take birth. They are endowed with the eight primary siddhis at birth.

Buddhism

The Svetasvatara presupposes a siddha body.