A definition of espresso along technical lines is perhaps least ambiguous: Espresso is coffee brewed from beans roasted medium to dark brown, with the brewing accomplished by hot water forced through a bed of finely-ground, densely-compacted coffee at a pressure of approximately nine atmospheres. The resulting heavy-bodied, aromatic, bittersweet beverage is often combined with milk that has been heated and aerated by having steam run through it until the milk is hot and covered by a head of froth.
To extend the technical definition somewhat, we might say that espresso is an entire system of coffee production, a system that includes specific approaches to blending the coffee, to roasting it, and to grinding it, and that emphasizes freshness through grinding and brewing coffee a cup at a time on demand, rather than brewing a pot or urn at a time from pre-ground coffee and letting the result sit until it is served.
