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.

Ahimsa is a primary Dharma (moral duty) widely practiced in ancient India. All Indian religions prescribes non-violence as the primary virtue. There is a vast literature available on Ahimsa. We could not find any spiritual text that does not describe Ahimsa. It is the highest morality of mankind.

Though the Sanskrit word has been the part of English Language since the late 19th Century, it became popular only when Mahatma Gandhiji made the non-violent protest against the then British Government of India which eventually leads to the independence in the year 1947.

Let us delve into this concept of non-violence a little deeper.

Ahimsa Meaning
The Sanskrit word Ahimsa is the antonym of Himsa. Himsa means harmfulness and causing pain and grief to other beings. Hence Ahimsa means harmlessness or non-violence. Simply put, it is absence of violence.

But the indicated meaning goes beyond non-violence. It includes relieving the sufferings of other beings: feeding the poor, treating the pains and sufferings of the diseases. Showing love and kindness is the real meaning of Ahimsa. Love and kindness should be ingrained in thoughts, speech, and deeds.

Ahimsa as the yogic virtue
The primary virtue to be practiced for the attainment of yoga is non-violence. This is why it has become the foremost of the Yamas of Yoga Sutra of Patanjali and most of the Yoga Upanishads.

Interestingly, Yoga Tattva Upanishad classifies Non-violence under Niyama and calls it as the foremost Niyama: Ahimsa Niyamesvega Mukhya.

Here is the comprehensive article on Ahimsa