The balance of a wine is a complex matter which wine professionals are still investigating, so as to be able to produce a system which can accurately assess all the elements which contribute to the organoleptic qualities of the product.
One of the greatest oenologists, the French Peynaud, summarised the balance of a wine thus, 'Alcoholic strength, smoothness, acidity, tannins; these being the main components of wine, the end result is the sum of them.'
The alcoholic strength is responsible for the heat of a wine, the body, which confers smoothness and roundness and counteracts the acidity and astringency. The smoothness is due to the sugars and glycerins, substances which mask bitterness and counterbalance acidity, conferring smoothness and roundness. The acidity is responsible for the freshness and liveliness of the wine, reinforcing the strength of the tannins and increasing the impression of bitterness. The tannic content (a key element in red wines) can give sensations of more or less bitterness (more or less pleasant) and reinforce the acidity.
In effect the balance of a wine is the result of these elements which combine, reinforce one another, hide one another or cancel one another out.