What Is Yuzu?
Yuzu is an East Asian citrus fruit, roughly the size of a small orange, with a highly aromatic, tart flavor often described as a cross between a mandarin, grapefruit, and lemon.
More detail
Yuzu is grown mainly for its zest and juice rather than eaten as fresh fruit, it's quite seedy and sour on its own. It's prized specifically for its intensely fragrant peel, which carries most of its distinctive floral, citrusy aroma.
Where it's used
Widely used in Japanese and Korean cooking, yuzu kosho (a fermented chile-yuzu paste), ponzu sauce, yuzu tea, and as a zested garnish over both savory dishes and desserts. It has become popular internationally in high-end cooking and cocktails over the past couple of decades.
Substitutes
A mix of lemon, lime, and a little mandarin orange juice/zest approximates yuzu's flavor profile reasonably well when it isn't available.
FAQ
Can I eat yuzu like a regular orange?
It's technically edible but very sour and seedy, so it's almost never eaten out of hand, its juice and zest are what's used in cooking, not the whole fruit as a snack.
Why is fresh yuzu hard to find outside Japan?
It has a short harvest season and doesn't ship or store as easily as more common citrus, so most markets outside East Asia carry bottled yuzu juice or yuzu kosho rather than the fresh fruit.