A sour Japanese citrus fruit about the size of a tangerine, with a thick bumpy green or orange rind. Like kaffir limes, yuzu are not eaten out of hand - they're much too sour - but the rind and juice are prized for cooking and perfumery.
The rind is very aromatic and slightly bitter; in Japan it's often simmered in clear soups to add a wonderful fragrance, or slivered and used as a garnish. Yuzu juice is used to enhance the flavour of many dishes; I recently had it with in a sauce over a delicious fusion shrimp sashimi and in a dessert soup with a semifreddo, which is how I heard of this fruit. As yuzu is only available in Japanese markets, lemons can be subsituted.
gn0sis tells me yuzu is sometimes referred to as Japanese citron.