The Specialty Coffee Association of America describes the definition of an espresso.
“Espresso is a 45ml (1.5 oz) beverage that is prepared from 7-9 grams of coffee through which clean water of 192 -198 F (88-92 c) has been forced at 9-10 atmospheres of pressure, where the grind of coffee has made the brewing “flow”* time approximately 22-28 seconds. While brewing the flow of espresso will appear to have the viscosity of warm honey and the resulting beverage will exhibit a thick dark gold cream foam (“crema”) topping. Espresso is usually prepared specifically for, and immediately served to its intended customer. ”
* Brewing “flow” is counted from the point in time that the espresso begins to flowing from the portafiller sprouts . Many espresso coffee machines have a distinct “pre-infusion” cycle where a small quality of water is pumped into the grounds and allowed to sit for several seconds before the pump is reactivated to finish the brew. The prescribed method of “timing” shots when the flow begins to appear at the portafiller sprout can be universally applied for all espresso coffee machines, no matter what the pre-infusion cycle.