Ahimsa is one of the yamas and means non violence. One practicing ahimsa would refrain from hurting others and oneself physically or psychologically. Ahimsa applies to all living things, even animals and plants, hence why vegetarianism is preferred in Yoga and is considered sattvic meaning in the mode of goodness(see [wiki]gunas[/wiki]). However, different yogic traditions have varying degrees of strictness on how to practice this. In Hinduism ahimsa is interpreted as non-violence is always preferred, but some violence is permissible in exceptional circumstances such as self defense or war, but only as a last resort. Hinduism considers vegetarianism optional, but emphasises it as being a more ethical practice. In Buddhism, ahimsa is interpreted as violence in general is not permissible, but the consumption of meat is not strictly prohibited in all Buddhist traditions. Jainism has the most strict interpretation of ahimsa where violence is not permissible under any circumstance whatsoever, and one must be ever mindful of even stepping on insects or breathing in microrganisms. This is the reason why Jain monks often wear a mask to cover their mouth and carry a broom with them to sweep insects aside.
Ahimsa is the first and most important of the yamas (restraints). It is the cardinal virtue upon which all others depend.
Refraining from causing harm to others, physically, mentally or emotionally.