Degree of oxidation

Green tea, oolong and black tea can all be made from the same plant. The difference is how far oxidation is allowed to go.

Heat applied straight after plucking stops it entirely and gives green tea; halting part-way gives oolong; taking it far gives black tea. Black tea is conventionally called 'fully fermented', but that does not mean oxidation runs to 100%. Teas such as pu-erh, where microbial action plays a part, form a separate category.