Krausen

A beer with heavy krausen overflowing through a blow-off tube. (right)

Kräusen, also spelled "kraeusen" or "krausen", pronounced "KROY-zen", is a beer-brewing term that has two definitions in that context.

1. A method to carbonate beer in which wort is added to the fermented/finished beer to carbonate.
2. The foamy, meringue-like head that develops during the initial stage of beer fermentation.

The word comes from German for frizzy hair ("krauses Haar"). The foam buildup is attributed to yeast eating sugars and building up CO2 in the process.