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Tapas (tapas) in Sanskrit means "heat". In Vedic religion and Hinduism, it is used figuratively, denoting spiritual suffering, mortification or austerity, and also the spiritual ecstasy of a yogin or tāpasa (a Vriddhi derivative meaning "a practitioner of austerities, an ascetic"). In the Rigveda, the word is connected with the Soma cult. The adjective tapasvin means "wretched, poor, miserable", but also "an ascetic, someone practicing austerities".
In the yogic tradition it is the fire that burns within that is needed for the sanyasi to achieve the very difficult goal of enlightenment, to foster self control, one mindedness and focus, simplicity, wisdom, integrity. It is used to develop and discipline the body, mind and character; control of mind; and ridding oneself of all desires - through discipline of body, correct speech, telling only the truth, correct thought, non violence, correct action, love for all, devotion to god, developing the ability to remain tranquil and balanced in every situation, act without any selfish motive or thought of reward, with an unshakable faith in god.
Sanskrit tapasyā (neuter gender), literally "produced by heat", refers to a personal endeavor of discipline, undertaken to achieve a goal, accompanying suffering and pain. Earliest reference of this word is to be found in the Rgveda-7.82.7a, where it is used in the sense 'pain, suffering' (Monier-Williams). It is usually applied in religious and spiritual terms, but can be applied to any field or context. One who undertakes tapas is a Tapasvin. From tapas the more widespread word tapasyā was derived, which is used in all three genders and was mentioned in Katyayana-Shrauta-Sutra, Baudhayana's Dharma-shashtra, Panini-4.4.128, etc. Rigveda has dozens of references to words derived from 'tapas' which indicate that "suffering, austerity" was its meaning from the Rigvedic times.
Monks and gurus in Hinduism, Buddhism and Jainism practice tapasya as a means to purify and strengthen their devotion to God, practice a religious lifestyle and obtain moksha, or spiritual liberation.
Tapas may be the striving for nirvana, or moksha. It may also be striving for perfection in a particular sport, field of knowledge or work. Tapasya may also be undertaken as penance, to liberate oneself from the consequences of a sin or sinful activities, or karma. But it will be wrong to equate Tapasya with penance, as done by some scholars. Tapasya can be done for a penance but it is definitely not a penance. Main motive of Tapasya can vary from spiritual gain, penance and even material gain.
David Frawley (in his book "Yoga and Ayurveda") defines Tapas as the spiritual transformation in which Tejas transforms Ojas to produce Prana. He defines Prana, Tejas and Ojas to be the subtle aspects of Vata, Pitta and Kapha (Doshas)respectively.
Tapasya is closely associated with meditation, fasting and the practice of yoga. Meditative tapas involves focusing entirely upon God, the Supreme Brahman and ignoring all environmental, artificial and other provocations or distractions. In the purest state of meditation, no thought save that of God must occupy the processes of the mind.
A tapasvin also practices brahmacharya, endeavoring to control all his or her biological instincts, functions and senses. Tapasvins reduce consumption of food and drink steadily, using their mental, intuitive force to reduce their biological needs. Ahimsa and vegetarianism, pure non-violence towards all living beings is practiced to eliminate anger, destructive impulses and avoid the foolishness of hurting others.
Fasting is accompanied by avoiding all cooked foods, especially spices and meats. Only fruits and roots are considered acceptable, and one may strive to reduce the quantity one has to consume.
Yoga and vows 
Yoga is closely linked with tapas. The disciplined and concentrated practice of yogic arts and exercises may be considered tapas by itself.
In the ancient scriptures of Hinduism, Jainism and Buddhism holy men, women and heroes undertake tapas to obtain a spiritual goal of realization, or salvation. Often tapas is undertaken to win a boon by pleasing a God, and many other times it is undertaken to increase one's pious characteristics.
Famous Ancient Tapasvins 
- Ravana: according to the Ramayana, as a young man Ravana undertakes a terrible penance, lasting over 1000 days to please Lord Shiva. When Shiva does not appear before him, Ravana begins to cut off his head, wherein a new head takes form out of his yogic power. He continues meditating again for a thousand days, then cutting another. When he is about to cut for a tenth time, Shiva appears. He grants Ravana's request for immeasurable strength and knowledge of weapons.
Ravana then undertakes another penance for 1000 days, endeavoring to please Lord Brahma. Brahma tells Ravana that he cannot grant him the immortality he desires, for none of the created are immortal. But Ravana obtains invulnerability against all celestial beings and living creatures, save man and monkeys.
- Vishwamitra: king Kaushika undertakes a heavy penance, fasting and meditating for thousands of years to become the equal of Guru Vasishta, a Brahmarishi. He steadily rises to become a Rajarshi, or a royal saint after a thousand years, but is not satisfied.
Even harder penance wins him the status of rishi, and rising to the brahmin order from the kshatriya order. But that is not enough for him. He strives harder to control his sensual passions, including the sexual urge which ruined his tapas when he consorted with the apsaras Menaka, and the anger which led him to attack Vasishta and turn another apsaras Rambha into stone.
After over 10,000 years, Lord Brahma rewards Kaushika's fearsome penance with the title of Brahmarishi, the highest of all brahmins and holy men, and equal to Vasishta. However now, Kaushika has dissolved his anger as a result of his tapas, and is named Vishwamitra, meaning Friend of the Universe, prepared to help anybody who sought his help. During his journeys, several times he gives all his TapoShakti (Energy gained after doing severe Tapas) to several people who sought his help and who he deemed as worthwhile. As a result, he earned the name Vishwamitra (Vishwam means Universe and Mitra means Friend).
Bringing Ganges back to Earth was a near impossible task and required many years to be spent in tapas and prayer. The Kosala kings of successive generations could not do this while managing their duties as kings. As a result, the sins of the thousand princes multiplied in their destructive energy, and began resulting in natural disasters. The kingdom began to lose its peace and prosperity, and by the time Bhagiratha ascended the throne, he found it impossible to attempt to govern in this situation, that had only one solution.
Turning over the kingdom to trusted ministers, Bhagiratha set off to the Himalayas to perform an arduous tapas in the extreme climate. For one thousand years, he performed an excruciatingly harsh penance to please Lord Brahma. At the end of the thousand years, Brahma came to him and told him to ask for anything. Bhagiratha asked Brahma to bring down the river Ganges to earth so that he may perform the ceremony for his ancestors.
Brahma asked Bhagiratha to propitiate Lord Shiva, for only He would be able to break the Ganges' fall. It was the largest river, and it would be impossible for anyone save Him to contain the destructive impact of this event.
Bhagiratha performed tapas for Lord Shiva, living only on air. The compassionate Shiva appeared only after a year's penance, and told Bhagiratha he should not have to perform tapas to accomplish a noble goal such as this. He assured Bhagiratha that he would break Ganges' fall.
Buddhism and Jainism 
The meditation undertaken by Siddhartha Gautama founder of Buddhism and Mahavira, the 24th Tirthankara of Jainism, earned Mahavira the exalted state of Moksha. Their tapas, undertaken to obtain salvation from a sinful, disturbing and violent world, is the foundation of Mahavira's teachings. However, Gautama realized that tapas alone were not enough to gain Enlightenment, and continued his search.
To this day, Buddhist and Jain monks keep rigid discipline, stringent vows and persistent study of their religion. Jain monks are often witnessed to take immense care by wrapping their feet in thick, soft cloth to avoid killing insects, as per the absolute non-violence taught by Mahavira.
Modern Tapasvins 
Modern Hindu mendicants pursue tapas - meditation and study of religion in ashrams across India and the world. Many hundreds of monks and mendicants base themselves around the holy sites of Hinduism, or in hermitages around the Himalayas to observe their vows and penance in as religious an environment as possible.
Exhausting or Violating Tapas 
One may exhaust all the merit or virtuous credit one has earned through tapas if one indulges in actions contrary to the principles and discipline of tapas, such as indulging in sense pleasures, or committing a sin or a crime.
Tapas may be violated if an outside influence or the tapasvin distracts himself or herself, successfully causing a break in the spiritual concentration or impurifying the environment in which the tapas is conducted.
Tapoloka is [situated] by 80,000,000 yojanas above Janaloka, and above Tapoloka by 120,000,000 yojanas is Satyaloka [abode of Brahma].
See also 
- Hinduism, Buddhism, Jainism
- Meditation, Nirvana, Brahmacharya, Moksha
- Satyagraha, Gandhism
- Monnier Williams Sanskrit Dictionary p.437
- "Monier Williams Online Dictionary" (in (German)). Sanskrit-lexicon.uni-koeln.de. 2011-11-26. Retrieved 2012-03-19.
- "Yoga and Ayurveda". MotilalBanarsidass.