In the United States, “Americano” is used broadly to mean combining hot water and espresso in either order, but in a narrower definition it refers to adding water to espresso (espresso on the bottom), while adding espresso to water (espresso on the top) is instead referred to as a long black.
The name derives its origins from WW2 when American GIs in Europe would pour hot water into espresso to approximate the coffee to which they were accustomed. It was then adapted in the United States by large chains who sought to create a marketable fusion of drip coffee and espresso to sell to a mass market.
Americanos – particularly short, long black style Americanos – are also used within artisanal espresso preparation for beans that produce strong espresso. This is particularly used for single origin espresso, where many find that undiluted espresso shots can prove overpowering. This is particularly used of lighter coffees and roasts not generally associated with espresso, such as beans of Ethiopian or Sumatran origins. For this preparation, generally a ratio of 1:1 espresso to water is used, to prevent excess dilution, with the espresso pulled directly into a cup with existing water to minimize disruption to the crema.

