Unagi is the Japanese word for freshwater eel, especially the Japanese eel, Anguilla japonica (not to be confused its saltwater cousin, which is called anago). The best are caught wild rather than bred in eel farms, with the ideal size between 30 and 50 centimeters. Fancy unagi restaurants keep tanks full of live eels, and they don't begin preparing your eel until after you've ordered.
It is said that unagi has been consumed in Japan for thousands of years. Because it is rich in protein, vitamins A and E, and so on, some people believe that unagi gives them stamina. For this reason, Japanese people eat the eel most frequently during the hottest time of the year in Japan.
It is, therefore, a Japanese custom to eat unagi on Doyo-no-ushinohi (the Day of the Ox during the Doyo period) in summer sometime between mid-July and the early part of August.
Filleted and deboned, unagi is commonly glaze-grilled, a process that makes it a dish known as unagi-no-kabayaki. It is the skewered and grilled with sweet basting sauce and is typically served over steamed, white rice. Vacuum-sealed unagi-no-kabayaki is often available at Asian grocery stores.
Well-prepared unagi combines a rich flavor, a bit like pate, with an appetizing texture, crisp on the outside but succulent and tender on the inside. The cooking process is what makes the eel crisp and tender: Unagi-no-kabayaki is cooked differently in eastern and western Japan. In the eastern part of the country, it is generally steamed after being grilled to remove excess fat, then seasoned with a sweetish sauce, and then it's grilled again. In the western part of Japan, in the Kansai area (around Osaka) unagi is not normally steamed before grilling but is grilled longer, burning off the excess fat. Unagi-no-kabayaki in eastern Japan is more tender than the same dish in western Japan and has an even crisper skin.
The ingredients in the sweet basting sauce are important to the final taste of the unagi, and different restaurants maintain their own secret recipes. The quality of the charcoal used is also important: The best charcoal is made from hard oak from Wakayama in central Japan, and the aromatic smoke adds a special flavor to the eel as it's grilling.
Unagi also can be used as an ingredient in other Japanese dishes like unagidon in which the eel is sliced and served on a bed of rice.
Sushi made with unigi is also pretty common fare. The sushi version is called unakyu.