What do you call the layer of cream that forms on top of espresso?

Crema is the correct term for the thin layer of golden-brown foam that forms on top of a freshly brewed espresso. This layer is a combination of oils, emulsified air, and microfoam that results from the high pressure and temperature during the espresso extraction process. The presence of crema is often used as an indicator of a high-quality espresso, suggesting that the coffee beans were freshly ground and the extraction process was executed properly. It enhances the coffee's aroma and contributes to its overall flavor profile, making it a key characteristic of a well-prepared espresso.

The other terms listed refer to different aspects of coffee preparation or other types of beverages. Foam and froth are generally used to describe the airy texture created by steamed milk, whereas scum typically has a negative connotation and refers to an unwanted layer of impurities or residue in liquids.