Wiktionary
a. Having two shots of espresso. n. Two shots of espresso.
Wikipedia
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 multiplier, 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).
Usage examples of "doppio".
Pazientammo diciotto anni, subimmo il doppio giogo, egualmente esacrato, dello straniero e del prete.
Our armies besieging Doppio castle, the Hungarian ambassador kissing the feet of Our Sultan, Our Prophet ascending through the seven heavens, these are of course all inherently happy scenes, but rendered by Butterfly, they become flights of ecstasy springing from the page.