Why do you call Italian espresso “espresso shot”?!

Why do you call Italian espresso "espresso shot"?!
Why do you call Italian espresso “espresso shot”?!

A “shot” refers to the amount of liquid you can drink at once. In this context “shot” of expresso means approximately one ounce of espresso (according to the Italian standards, a volume of 25 ml plus or minus ten percent.)