Crema
What is crema in coffee?
Crema is the golden-brown foam on top of an espresso. It forms when high pressure dissolves carbon dioxide and coffee oils into the liquid, which then foam as the pressure is released. Fresh coffee makes more crema, but crema alone does not mean an espresso tastes good.
Very fresh or Robusta-heavy coffees produce thick crema. Compare the foam on Turkish coffee, called köpük, which forms by slow heating rather than pressure.