If you love to talk about what a strict vegan you are, your friends might start calling you a fundamentalist, or a person who sticks very closely to a particular set of principles.
The word fundamentalist is used most often to talk about religion. As an adjective, it describes a very strict, literal interpretation of a religious text or set of beliefs, and the noun means a person who holds those firm, often extreme, beliefs. Fundamentalist is a relatively recent term, invented in the 1920s in the United States to describe a Protestant movement during that era.
