It is one of the 5 restraints. Ahimsa a non-violence. It is of 3 types:
1. Mental: Should not think to harm anyone. The thought of violence will slowly transform into revenge. So first the thoughts of violence should be controlled.
2. Verbal: Should not hurt anyone by abusing, scold, curse or insulting or disobey rudely.
3. Physical: Should not create any injury directly or indirectly. Injury either by beating, causing injuries, killing (animal hunting and in such business, consuming).
Conclusion: When the mental ahimsa is attained, it is easy to attain the other two (verbal and physical ahimsa). Complete ahimsa means not to take revenge even if anyone causes harm (either mentally, verbally or phytsically).