Doppio in espresso is a double shot, extracted using a double coffee filter in the portafilter. This results in 60 ml of drink, double the amount of a single shot espresso. More commonly called a standard double, it is a standard in judging the espresso quality in barista competitions. Doppio is Italian, meaning "double".
A single shot of espresso by contrast is called a solo ("single"), and developed because it was the maximum amount that could practically be extracted on lever espresso machines. Today the doppio is the standard shot, and because solos require a different filter basket, solo shots are often produced by making ("pulling") a doppio in a two-spout portafilter, but only catching one of the streams (the other stream may be discarded or used in another drink).