Carbonation

From Fermented Beverage Production:

In the United States, a wine is legally considered a sparkling wine if the carbon dioxide content is above 3.92 grams per liter. Wines with carbon dioxide levels below this point are legally considered still wines, even though they may have perceptible carbon dioxide levels and may still taste "fizzy." International recommendations from the OIV place a maximum carbon dioxide level for still wines at 2 grams per liter.