Double, in the context of espresso brewing, refers to the volume of coffee pulled at one time. A double shot of espresso is pulled from approximately 14 grams of coffee grounds. The filter basket used for pulling it, is also called a double, and it can hold 14 grams of coffee grounds. The double espresso shot is also called a doppio, the Italian word for double. 

A doppio has approximately 2 oz. (60 ml), of beverage, the double of the solo, (the standard espresso volume). The 2 oz. is the standard extraction for a normale. We can also extract a doppio ristretto or a doppio lungo, and the volume will modify accordingly.