Wasabi

Wasabi (ワサビ（山葵）?, originally 和佐比; Wasabia japonica, Cochlearia wasabi, or Eutrema japonica) is a member of the Brassicaceae family, which includes cabbages, horseradish, and mustard. Known as "Japanese horseradish", its root is used as a condiment, it has a pungent flavour which you can feel in your mouth and nose when you eat it.

With Sushi
Wasabi is often served with sushi or sashimi, usually accompanied with soy sauce. The two are sometimes mixed to form a single dipping sauce known as Wasabi-joyu. Due to the expense and difficulty of cultivating wasabi, a very widely used substitute (imitation wasabi) is a mixture of (western) horseradish, mustard, and green food coloring; in American sushi restaurants this is generally referred to as "wasabi", while genuine wasabi, which is rarely available, is referred to as "fresh wasabi". Covering wasabi until served preserves flavor, and for this reason, sushi chefs usually put the wasabi between the fish and the rice.

Misc. Uses
Wasabi is generally sold either in the form of a root which is very finely grated before use, or as a ready-to-use paste (either real wasabi or a mixture of horseradish, mustard, and food coloring), usually in tubes approximately the size and shape of travel toothpaste tubes. In some restaurants the paste is usually prepared as needed by the customer using the root and a grater directly; once the paste is prepared, it will lose flavor within 15 minutes. In sushi preparation, Fresh leaves of wasabi can also be eaten and have some of the hot flavor of wasabi roots.