From Wikipedia, slightly edited
Ahimsa is a Sanskrit term meaning non-violence or non-harmfulness. It is an important tenet of the religions that originated in ancient India, being found in Hinduism, Buddhism and especially Jainism. Ahimsa is a rule of conduct that bars the killing or injuring of living beings. It is closely connected with the notion that all kinds of violence entail negative karmic consequences. The extent to which the principle of non-violence can or should be applied to different life forms is controversial between various authorities, movements and currents within the three religions and has been a matter of debate for thousands of years.
Though the origins of the concept of ahimsa are unknown, the earliest references to ahimsa are found in the texts of historical Vedic religion, dated to 8th century BCE. Here, ahimsa initially relates to "non-injury" without a moral connotation, but later to non-violence to animals and then, to all beings. The idea emerges again in the Hindu texts Mahabharata and Manu Smriti, where ahimsa is said to be merited by good Karma. Though meat-eating and slaughter of animals are criticized by some Hindu texts, other texts present counter-arguments in support of hunting and ritual sacrifice.
Ahimsa in Jainism emphasizes vegetarianism and bans hunting and ritual sacrifice. Jains go out of their way so as not to hurt even small insects and other minuscule animals and make considerable efforts not to injure plants in everyday life as far as possible. In accordance to this policy, eating of some foods, whose cultivation harms small insects and worms as well as agriculture itself, is to be abstained from. Violence in self-defense, criminal law, and war are accepted by Hindus and Jains. Though ahimsa is not used as a technical term in Buddhism unlike the other two religions, it condemns ritual sacrifice and war and moral codes emphasize the importance of not taking life.
Ahimsa is imperative for practitioners of Patañjali's "classical" Yoga (Raja Yoga). It is one of the five Yamas (restraints) which make up the code of conduct, the first of the eight limbs of which this path consists. In the schools of Bhakti Yoga the devotees who worship Vishnu or Krishna are particularly keen on ahimsa. Another Bhakti Yoga school, Radha Soami Satsang Beas (Sant Mat) observes vegetarianism and moral living as aspects of "ahimsa." Ahimsa is also an obligation in Hatha Yoga according to the classic manual Hatha Yoga Pradipika (1.1.17).
In the 19th and 20th centuries, prominent figures of Indian spirituality such as Swami Vivekananda , Ramana Maharishi , Swami Sivananda  and A.C. Bhaktivedanta  emphasized the importance of ahimsa. Mahatma Gandhi promoted the principle of ahimsa very successfully by applying it to all spheres of life, particularly to politics. His non-violent resistance movement satyagraha had an immense impact on India, impressed public opinion in Western countries and influenced the leaders of various civil rights movements such as Martin Luther King Jr. In Gandhi’s thought ahimsa precludes not only the act of inflicting a physical injury, but also mental states like evil thoughts and hatred, unkind behavior such as harsh words, dishonesty and lying, all of which he saw as manifestations of violence incompatible with ahimsa. Sri Aurobindo criticized the Gandhian concept of ahimsa as unrealistic and not universally applicable; he adopted a pragmatic non-pacifist position, saying that the justification of violence depends on the specific circumstances of the given situation.
A thorough historical and philosophical study of ahimsa was instrumental in the shaping of Albert Schweitzer's principle of "reverence for life". Schweitzer criticized Indian philosophical and religious traditions for having conceived ahimsa as the negative principle of avoiding violence instead of emphasizing the importance of positive action (helping injured beings).
References Religious Vegetarianism, ed. Kerry S. Walters and Lisa Portmess, Albany 2001, p. 50-52.
 Ramana Maharishi: Be as you are
 Swami Sivananda: Bliss Divine, p. 3-8.
 Religious Vegetarianism p. 56-60.
 Tähtinen p. 116-124.
 Walli p. XXII-XLVII; Borman, William: Gandhi and Non-Violence, Albany 1986, p. 11-12.
 Tähtinen p. 115-116.
 Schweitzer, Albert: Indian Thought and its Development, London 1956, p. 80-84, 100-104, 110-112, 198-200, 223-225, 229-230.
 Patañjali: Yoga Sutras, Sadhana Pada 30.
 Tähtinen p. 87.
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