Wasabi is a root vegetable known in English as Japanese horseradish. It is most famous in the grated form of a green paste used as condiment for sashimi (raw seafood) and sushi. However, wasabi is also used for many other Japanese dishes. In supermarkets, wasabi is widely available as a paste or in powder form. Wasabi powder has to be mixed with water to become a paste. Wasabi has a strong, hot flavor which dissipates within a few seconds and leaves no burning aftertaste in one's mouth.
Many "wasabi" powder and paste products that are available in supermarkets (and even some restaurants) contain only very little or no real wasabi at all and are made of colored horseradish instead. This is due to the fact that cultivation of real wasabi is relatively difficult and expensive.