纯 (chún) 'pure' is what distinguishes this from regular 素食主义 (sùshízhǔyì) 'vegetarianism'. A 素食 vegetarian may still eat eggs and dairy; a 纯素食 vegan eats no animal products at all. A vegan person is 纯素食主义者 (chúnsùshízhǔyìzhě) or just 纯素者 (chúnsùzhě).
culture
Traditional Buddhist vegetarianism in China (素 sù) historically excluded the 'five pungents' (五荤 wǔhūn — garlic, onion, leek, etc.) but did sometimes include dairy. Modern Western veganism mapped onto Chinese as 纯素 'pure 素'.
