Short definition
When a wine's acidity, sweetness, tannin, alcohol, and fruit are in proportion with no single element dominating.
A balanced wine feels complete. The acidity does not make you wince, the alcohol does not burn, the tannins do not strip your gums, and the fruit does not taste jammy. Each part supports the others. A $12 Côtes du Rhône can be perfectly balanced while a $200 Napa Cab might be out of whack if the oak overwhelms the fruit.