Oolong teadefies a fixed definition. It is typically described as “partially oxidized tea”, which is not wrong, but it’s not a complete picture. The oxidation level does not distinguish it as oolong tea.
It’s their similarly shared process of elaborate transformation (withering, shaking, pan-firing, rolling, drying, baking) that put these otherwise disparate group of teas under the same name.
Note that the proper pinyin spelling is “wulong” tea, but it is more common in English to see it spelled “oolong" tea and they mean the same thing.
