Ahimsa means non-harmfulness (harmlessness), to not wish harm to any living creature — not even to any lifeless object.
This doesn’t mean to not harm or kill, which can sometimes be inevitable, such as when one eats meat or even plants, or accidentally steps on bugs. Ahimsa is about the intent, rather than the action itself. It is an attitude of universal benevolence.(1)
In Orthodox Hinduism
The Hindu mystic Patanjali wrote a scripture called the Yoga Sutras, where he outlines yamas (restraints, or what one should not do) and niyamas (observances, or what one should do). Ahimsa is the first of the yamas. Patanjali says that once ahimsa is mastered, even wild animals and ferocious criminals will become tame and harmless in our presence.
Traditionally ahimsa is taken to mean that a person should not kill. This is why vegetarianism is so widespread in India.(2) Some Hindus believe that one should not kill or harm anything even to save one’s own life. Some sects in India go to great measures to follow this, such as boiling water to avoid killing germs when they drink it. (3)
The term was popularized in modern times by Mahatma Gandhi. By non-violent resistance he led India to political emancipation from Britain.(4)
The Deeper Meaning of Ahimsa According to Swami Kriyananda
Ahimsa, rightly understood, is the ultimate weapon; it turns one’s enemy into a friend, thereby banishing the possibility of further conflict. In the practice of yoga, it is important to understand that the same life flows in the veins of all creatures.
What Patanjali referred to, essentially, was the attitude of the mind, rather than the literal acts of the body. It is one’s attitude that can either lead him toward liberation, or hold him in greater bondage. An attitude of harmlessness (and its corollary, a feeling of universal benevolence) is what is meant by ahimsa. It is not possible in any case kill anyone: The soul is immortal. What is possible, however, by wishing harm to another living being, is develop a consciousness of death, which causes harm to the perpetrator.
The principle of ahimsa must be understood in subtle ways, not only in gross. To harm anyone in the slightest way, even by disrespect, will harm the person doing the action as well as the one receiving it. The perfect practice of ahimsa, then, is very rare. For though not many people would actually kill their fellows, it is common to find people slashing at one another with angry words, or with contemptuous glances.(3)