So... What is Espresso?
Is it a roast?
a type of bean?
Is it a grind?
or a method?
Well, the answer is… It’s kind of all of those things, but let’s start with a quick definition from Webster dictionary which states:
es·pres·so /eˈspresˌō/ noun is strong black coffee made by forcing steam through ground coffee beans. From Italian (caffè) espresso, literally ‘pressed out (coffee)’.
So in the purest sense, espresso is simply a coffee brewing method where steaming hot water is forced through very finely ground coffee beans. The conception and subsequent invention of this brewing technique can be traced back to the late 19th century, when European cafe culture was going strong and there was an incentive to speed up the brewing process.
The patent for the espresso machine, “the new steam machinery for the economic and instantaneous confection of coffee,” is ultimately attributed to Angelo Moriondo of Turin, Italy.
While it sounds simple enough, there’s actually quite a lot of science involved in achieving the right combination of temperature, pressure, and grind to achieve the perfect pull. To be more precise for our scientific friends, espresso is formed when hot water at 88°-93°C (190°-200°F) passes under a pressure of nine or more atmospheres through a seven-gram (.25 oz) cake-like layer of ground and tamped coffee resulting in a concentrate of not more than 30 ml (one oz) of rich, velvety, coffee.
As a result of the pressurized brewing process, the flavors and chemicals in a typical cup of espresso are unique and very concentrated. Let’s dive a bit deeper and look at the molecular level. The substantiating difference between espresso vs other coffee can be attributed to the emulsion of oil droplets, which preserve some of the aromatic compounds that are lost in other brewing methods, topped by a layer of gas bubbles, or foam. The resulting dispersion is perceived in the mouth as creamy and preserves the strong flavor that is characteristic of espresso.
Is espresso a specific roast?
Technically, no. Authentic espresso coffee can be made with a wide variety of coffee beans and roast degrees.
In my research, it seems that through trials and tastings, aficionados agree that a darker roast yields a better espresso, but lighter roasts are popular in some regions.
Heating the raw, green coffee beans to different degrees, alters and releases certain compounds that yield different flavors. As a rule, lighter roasts are more acidic and botanical in flavor. As coffee beans roast longer, they becomes darker, less acidic, and more bitter. Darker roasts can also produce an oil layer on the outside of the bean that some swear enhances or intensifies the coffee flavor.
The preference towards darker roast for espresso is thought to stem from the bean acidity. Espresso is made at a lower temperature compared to filter coffee. When using lighter roasts, this lower temperature, as well as the forced pressure, results in coffee which is more sour. Using dark roasts eliminates acids from beans thus yielding a “better” overall taste for espresso. However, the darker the roast, the stronger the burnt flavor gets, the lower the caffeine, and sometimes, the more flat the resulting coffee... so individual preferences for acidity vs bitterness and mouthfeel all contribute.
Why Do I see coffee labeled as "Espresso Roast"?
Growing and roasting coffee is a science and an art with an expansive list of variables. First, consider that there are different bean species, different growing regions (each with seasonal variations from one year to the next), different farming and agricultural practices, and variable drying and storage processes. Then, consider that each roaster uses different techniques, machines, batch sizes, and additives, which results in an overwhelming sea of options for the consumer.
The term “espresso roast” was likely born from a need to make an easy to recognize option for consumers. As a rule, when you see a coffee labeled as espresso roast, it will be a very dark roast because that is the overwhelming preference.
But, play around with your coffee. By limiting yourself to only using “espresso roasts” in your machine, you may be missing out!
Is there an “espresso grind”?
Yes! What is more important than the roast that you choose for espresso, is the grind. Beans for espresso are ground much finer than for filter coffee. Again, this has to do with the method of extraction. Espresso machines use water under pressure, so it must have some resistance while penetrating coffee or else the coffee will be flat. A fine grind, between the consistency of flour and table salt, is the best for espresso. 
How much caffeine is in an espresso?
According to the Department of Agriculture, on average, 1 ounce of espresso contains approximately 63 mg of caffeine.
An average 8oz cup of coffee contains 95 to 128 mg of caffeine, which comes out to 12 to 16 mg per ounce.
So, one cup of coffee on average has more caffeine than one shot of espresso... But if you really want to amp up your coffee- add a shot or two of espresso to your cup.
https://www.smithsonianmag.com/arts-culture/the-long-history-of-the-espresso-machine-126012814/
https://www.illy.com/en-us/coffee/coffee-preparation/how-to-make-espresso-coffee