Oolong (sometimes pronounced 'wulóng') is a traditional Chinese tea ranging from 10% to 70% oxidation -- between green and black tea.
In Chinese tea culture, semi-oxidized oolong teas are collectively grouped as qingcha, literally: blue-green tea. It has a taste more akin to green tea than to black tea: it lacks the rosy, sweet aroma of black tea but it does not have the stridently grassy vegetal notes that typify green tea.
The best Oolong has a nuanced flavor profile. Oolong tea leaves are often processed and rolled into long curly leaves or into ball-like form similar to gunpowder tea. It is commonly served in Chinese restaurants, to accompany dim sum and other Chinese food.