Surprisingly, there is no real definition of espresso–there are certainly elements that the experts agree on, but there are no codified guidelines, no explicit recipes.
While this is true, I’d argue that it’s equally true of many foods. Recently I had an “apple pie” that was more like an apple cake. I also had a “burrito” that didn’t include cheese and salsa.
All of these foods didn’t meet my definitions, but you know what? That’s fine. Food and beverages never have strict rules. We as humans are inherently bad at describing taste and texture of the things we ingest, making such definitions difficult.
More confusingly, what was considered “espresso” a century ago ago wouldn’t satisfy most of us today. We might even spit it out. (Good thing I’m not a food historian, that’s got to be an impossible job!)
I’d argue that if espresso had a concrete definition, there’d be no room for improvement and no room for different styles and tastes. Who’d want to live in a world without variety?