Short definition
The traditional method of making sparkling wine with a second fermentation inside the bottle, creating fine bubbles.
A still base wine is bottled with a small amount of yeast and sugar. The yeast ferments inside the sealed bottle, producing CO2 that cannot escape — that is where the bubbles come from. The wine then ages on the spent yeast (lees) for months or years, gaining complexity and that signature toasty, brioche character. Champagne, Cava, Franciacorta, and English sparkling all use this process.