Oxidation or fermentation?
You have seen black tea called fully fermented and green tea called unfermented, and you have seen the same teas described as oxidised. Both are in print, sometimes in the same shop. Which word is the right one?
Short answer
Oxidation, for everything except puerh and heicha. Oxidation is enzymatic: the leaf's own enzymes act on its own compounds once the cells are broken open and the air gets in. Fermentation is microbial, and in tea it happens in one place, the damp pile that makes ripe puerh and the heicha piles built the same way. Calling black tea fermented is an old trade habit that predates the chemistry, and tea books still disagree about the word, which is why you have met it both ways. It is worth keeping straight, because the two point at different chemistry and at different behaviour once the tea is on your shelf.