In brewing a carboy is a glass or plastic container used for fermenting beverages such as wine, mead, and beer. It is fitted with a rubber stopper and a fermentation lock in the neck to prevent bacteria and oxygen from entering during the fermentation process.
During a small scale homebrew, it is recommended to use a primary carboy for fermentation. Once primary fermentation is complete, the beer can be either transferred to a secondary carboy for conditioning or it can be transferred directly to bottles for conditioning. This process of transferring is usually called racking.
Carboys can come in various sizes and volumes ranging from 3.8 to 24.7 liters. The term carboy itself usually refers to a 19 liter carboy, unless otherwise noted. A 4.5 liter carboy is commonly called a jug and a 57 liter carboy is usually called a demijohn.