What exactly is espresso?

Espresso is a concentrated coffee beverage brewed by forcing near-boiling water through finely-ground, tightly-packed coffee using high pressure — typically 9 bars (about 130 psi). The result is a small, intense shot (25–35ml) with a caramel-colored foam layer called crema. Espresso is the base for most coffee drinks: lattes, cappuccinos, flat whites, and Americanos are all built on espresso shots.