Short definition
A wine that has been overexposed to oxygen, losing its freshness and developing flat, stale, or nutty off-flavors.
Oxidation happens when wine gets too much air — from a faulty cork, an improperly sealed bottle, or simply being open too long. White wines turn darker (brownish). Reds lose their bright color and taste flat. The aromas shift from fresh fruit to bruised apple, sherry-like nuttiness, or vinegar. Some styles (Sherry, Vin Jaune) are intentionally oxidized. For everything else, it is a flaw.