Yogapedia defines ahimsa:
Ahmisa is a Sanskrit word meaning "non-violence." The term is derived from the root word himsa, meaning "to cause pain," and the prefix, a-, that means "not."
In a broader sense, ahimsa means "universal love and compassion." Practice of ahimsa involves refraining from causing physical and psychological pain to any living being. Ahimsa is forgiveness, divine love and sacrifice. Ahmisa, in short, is being non-violent in thought and action, in body and soul.
Ahimsa is one of the five yamas, which is the first "limb" of the great sage Patanjali's eightfold path (ashtanga) as described in the Yoga Sutras. Ahimsa is considered to be the most important of the five types of yamas because it represents the right attitude necessary to lead a moral life.
Ahimsa is the absence of violence in physical, mental and emotional forms.
The part about any living being covers animals, including those commonly exploited and raised to become meat or who are used for their milk, eggs, fur, skins or as beasts of burden.
Veganism is ahimsa. Learn to go vegan here.