If you drink coffee in America, there’s a good chance that you drink drip coffee, brewed by dripping boiling water over a basket of ground coffee and then served in a cup ~6-8 ounces in volume. If you drink coffee in Italy, there’s a good chance you drink espresso coffee, brewed by putting very finely ground coffee under extreme pressure and then served in a cup ~2-2.5 ounces in size (a single shot of espresso is ~1-1.5 ounces in volume). The end result is significantly different with the pressurized extraction of the espresso drawing out more oils and different flavors than drip brewing.
While at first glance, it seems like the two coffee preparation styles are so different that never the twain shall meet, there’s a sort of compromise between the two drinks. Because there are many places, like Italian cafes for example, where drip coffee is not kept ready and on hand, hot water will be added to a single or double shot of espresso to dilute it to the strength a drip coffee drinker is used to. The end result is a serving of coffee with the volume and caffeine level the recipient is used to, but with a different and richer flavor profile as a result of the extra coffee oils–as seen in the photo here.
The practice is believed to have originated during World War II when American soldiers would dilute espresso with hot water to make something that more closely resembled the coffee they were used to back home, thus the hybrid coffee became the “Americano”.