Espresso (and since the word is Italian, we're going to call it espresso, even though most modern dictionaries accept expresso as a variant) is a type of [ingredient=coffee coffee] produced by the action of pressurized, heated water on a finely ground bed of coffee. The combination of pressure, temperature and grind size produces a liquid with a foamy head of emulsified oils (called crema) on top of a concentrated, black liquid. The production of crema is the hallmark of espresso. 

While the beginnings of espresso date back to Italy in the late 19th century, it was in December 1901 that Luigi Bezzera first put theory into practice, patenting improvements in the coffee making process. In 1905, his patent was bought by Desiderio Pavoni, who founded the La Pavoni company to commercialize the technology. The goal of this new technology was to make coffee breaks shorter. Over the years, espresso machines have undergone a constant stream of technological improvement.

The original espresso machines were simple steam boilers attached to four group heads, which allowed for multiple brewing methods. In 1945, Achille Gaggia introduced a lever-driven piston machine where the barista would literally pull down a lever arm to pressurize hot water -- this is the origin of the phrase "pulling a shot". 

In 1961, the Faema company introduced the E61 group head (which is sometimes called Heat Exchanger, or HX). This development basically uses water to heat a massive (typically eight and a half pounds) chrome-plated brass grouphead sink, which promotes temperature stability, which is a requirement for qualty espresso. They also began to produce machines with pumps instead of pistons to drive the pressurization of water. 

Today, mosty commercial espresso machines use Dual Boiler/Heat Exchanger technology, with separate boilers for espresso and steam for milk. Most high end machines are fitted with PID (proportional–integral–derivative) controller, which allows for tenth of a degree Fahrenheit settings for boiler temperature. Today's espresso machines take a long time to heat to proper working temperature, but once they get there, very little will move them off their setpoint. 

With the rise of urbanization and Italian laws that controlled the price of coffee, provided it was drunk standing, espresso gained a foothold among the working class. It was then exported to the United States, starting in predominantly Italian neighborhoods. With the rise of Starbucks in the late 1980's and 1990's, espresso (typically in milk drinks) became a large part of America's coffee consumption. 

Traditionally, espresso was produced by four words, all starting with "M": Macchina, the espresso machine. Macinzaione, the coffee grinder. Miscela, the blend of coffee beans. Mano (Hand), the person "pulling" the shot of espresso. While super-auto machines exist (they grind beans, tamp them, pull the shot, and dispose of the spent coffee), they run counter to the ideals of the four "M"s.

A proper shot of espresso is 1.5 ounces of liquid made from 7-9 grams of fine ground coffee through which water at 200 degrees is forced. The entire process should take between 25 and 30 seconds. While brewing, espresso should have the look of warm honey. Proper espresso should have balance between sweetness, acidity and texture. These are fleeting, and commonly called God Shots. 

Some popular cocktails containing Espresso

  • Fernet Flip-out — Fernet Branca, Mezcal, Walnut Liqueur, Bitters, Peychaud's Bitters, Espresso, Whole egg, Agave syrup, Orange peel
  • Lazo — Rum, Grapefruit juice, Honey syrup, Espresso, Salt Solution, Grapefruit peel
  • Orinoco — Rye, Bitters, Whole egg, Espresso, Rich simple syrup 2:1, Coffee
  • Black Cherry — Sweet vermouth, Rye, Maraschino Liqueur, Bitters, Espresso
  • Un Café Va Bene — Cynar, Cassis, Espresso
  • Somnambulist — Brandy, Coffee liqueur, Irish Cream, Espresso
  • Rye Cappuccino — Rye, Chocolate bitters, Espresso, Cream, Rich demerara syrup 2:1
  • The Noble Beast — Bitters, Espresso, Whole egg, Rich demerara syrup 2:1, Coffee bean
  • Xocolatl Talisman — Blended Scotch, Drambuie, Bitters, Espresso, Agave syrup, Cayenne pepper, Coffee bean
  • Pittsburgh Coffee — Fernet Branca, Drambuie, Espresso