Ahimsa, literally non-injury, is the principle of not harming living beings. It is shared by Hinduism, Buddhism and Jainism, and the three traditions differ substantially in how far they extend it and how they reconcile it with the obligations of ordinary life.
Jainism develops it most rigorously and makes it the governing vow from which the others follow. The Acaranga Sutra argues from the observation that all beings desire to live and none wishes to be harmed; Umasvati's Tattvartha Sutra gives the systematic account. Jain practice extends ahimsa to the one-sensed beings held to inhabit earth, water, fire, air and plant bodies, which produces the tradition's characteristic disciplines: strict vegetarianism and the avoidance of root vegetables, the straining of water, the mouth-cloth worn by some mendicants, the sweeping of the path before sitting, and the traditional avoidance of agriculture as a livelihood for the devout, since ploughing kills. Jain laypeople have historically concentrated in trade and finance partly for this reason.
Hinduism lists ahimsa among the yamas, the restraints, in Patanjali's Yoga Sutras, where it stands first and is described as a great vow not limited by circumstance, class or time. It is also qualified in practice, and the tension is not concealed: the dharmasastra literature permits animal sacrifice in some ritual contexts, and requires the use of force by kings and by the warrior class. The Bhagavad Gita opens with exactly this problem, Arjuna refuses to fight, and the answer given is not that killing does no harm but that duty performed without attachment to its results does not bind the agent. The friction between the renunciant ideal and householder and royal obligation runs through the whole tradition and was never resolved into a single rule.
Buddhism makes abstention from taking life the first precept, and analyses the matter through intention, since the karmic weight of an act depends on the volition behind it. The Theravada monastic code forbids killing and permits meat that was not seen, heard or suspected to have been killed for the recipient; Mahayana traditions in East Asia largely adopted vegetarianism, arguing from sutras such as the Lankavatara.
The term's modern currency is largely due to M. K. Gandhi, who made ahimsa the basis of a political method and presented non-violence as an active force rather than a mere abstention. His reading was contested within India from several directions, and B. R. Ambedkar's criticisms of the wider programme were sustained; scholars have also argued that Gandhi recast a renunciant ideal as a mass political technique, which is a description rather than an objection. Through Gandhi the concept reached Martin Luther King Jr. and much of the subsequent history of non-violent resistance.