Often when talking about oolong tea you sometimes hear that a tea was 25% fermented, or that black tea is fully fermented. Sadly both those things are wrong to say, they are common usage due to a translation error from Chinese into English. But the correct word in both those situations is oxidized. To be sure we are on the same page here are the definitions of the two words, which most closely relate to the process tea goes through:

• Fermentation – a process in which an agent causes an organic substance to break down into simpler substances
• Oxidation – add oxygen to or combine with oxygen.

No when an oolong is oxidized, or a black tea is oxidized they are left out in open air to and the oxygen reacts with certain enzymes in the leaves. The unique thing about oxidizing tea leaves is history has told us that leaves are less likely to go bad the more they are oxidized. This is how black tea became so popular in England, as when fully oxidized the tea still tasted palatable after a long journey west.
So is it wrong to say that a tea is fermented? Not necessarily, as aged teas do undergo a fermentation process, in which the cell walls and other parts of the leaf degrade into sugars. So it is okay to say that 10 or more year old pu’erh or oolong is fermented. I should note that while this is labeled as fermentation it is similar to the fermentation of grains to produce beer, but no an aged tea is not alcoholic.
So now you know the difference between Oxidation and Fermentation and when to use them properly when referring to tea. Just to be sure get a well aged tea, and an ordinary black tea, and taste them side by side. The color might be similar but I’m sure the taste difference will astound you.