Disgorgement is the Champagne process in which sediment is expelled from the bottle neck.
During the second fermentation, yeast cells and other particles form a deposit called lees. Riddling gradually moves that sediment into the neck, where it can be collected near the temporary closure. In modern production, the neck is commonly chilled so the sediment freezes into a small plug. Removing the temporary cap allows the pressure inside the bottle to eject the frozen deposit.
After disgorgement, the producer may add a measured mixture of wine and sugar, called the dosage or expedition liqueur. This step determines the final sweetness category, such as brut or demi-sec, and the bottle is then fitted with its permanent cork and wire hood.
Disgorgement is not the same as riddling. Riddling moves the sediment; disgorgement removes it. Tirage refers to bottling the base wine with the ingredients needed for the second fermentation, while dosage happens after the sediment has been expelled.