Veganism may be defined as a way of living which seeks to exclude, as far as possible and practical, all forms of
exploitation of, and cruelty to, animals for food, clothing or any other purpose.
In dietary terms it refers to the practice of dispensing with all animal produce - including meat, fish, poultry, eggs, animal milks, honey, and their derivatives (e.g. gelatin).
Since yeast is neither animal nor plant, only a few vegans avoid eating yeast and thus only eat unleavened bread.
Abhorrence of the cruel practices inherent in dairy, livestock and poultry farming is probably the single most common reason for the adoption of veganism, but many people are drawn to it for health, ecological, spiritual and other reasons.