牛油果 (literally 'butter fruit') is the everyday colloquial name for avocado in mainland China. The formal/scientific name is 鳄梨 ('crocodile pear'), but it appears almost only on labels and in Taiwan, where 酪梨 is also common.
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牛油 means 'butter' and 果 means 'fruit' - think of avocado's rich, buttery flesh.
