Reduction is a chemical process that occurs without oxygen. In wine it can change the aromas of a wine, and is often associated with wine making and storing wines under screw cap.
Too much reductive character in the wine is not the best with flavours such as burnt match, onion, garlic, cabbage, nuttiness, cat’s pee and sweaty aromas occuring. Lower levels of reductive characters are thought by some to produce the mineral character in many top French wine as well as green pepper and black current notes in some wines.
So as with oxidation in wine a small controlled amount can be good, but too much is usually a problem. The rise of the New World fruit driven style of wines was based on not allowing the grapes and wine to contact oxygen, this is known as reductive wine making.
The cork industry has claimed that the lack of oxygen reaching the wine because of the near perfect seal from screw caps causes reductive sulphur like characters. Their has been several reports from wine competitions on alleviative sulphur levels detected on wines under screw cap.