Kabayaki (蒲焼) is a preparation of the unagi eel, sometimes extended to other fish, where the fish is split down the back (or belly), gutted and boned, butterflied, cut into square fillets, skewered, dipped in a sweet soy sauce-base sauce before broiled on a grill.
The same preparation is made of other long scaleless fish such as hamo (pike conger), dojō (loach), catfish, anago (conger eel), and gimpo (ギンポ) (Pholidae). One can also find canned products labeled as kabayaki-style sanma (Pacific saury).
Kabayaki eel is very popular as a nutritious invigorating food (a rich source of vitamins A and E, and Omega-3 fatty acids). A popular custom from the Edo period calls for eating kabayaki during the summer to gain stamina, especially on a particular mid-summer day called doyō-no ushi-no-hi(ja) (土用の丑の日), which can fall anywhere between 7/18~8/8 each year.
The eel kabayaki is often served on top of a bowl (donburi) of rice, and called unadon, the fancier form of which is the unajū, placed inside a tiered lacquered boxes called jūbako. It is also torn up and mixed up evenly with rice to make hitsumabushi (櫃まぶし), which is enjoyed especially in the Nagoya area.
Kantō vs. Kansai 
Broadly there are two schools of cooking the kabayaki. In the Kantō region (eastern Japan), the eel is slit down its back, and butterflied, so that there is a lighter-colored stripe of the belly down the middle of each fillet on the skin side. The long eel is cut into shorter squarer fillets and skewered. In Kanto, the skewered eel is first broiled plain into what is known as shirayaki(ja) (白焼き or 素焼き), then steamed, before being flavored and grilled again, and as a result turn out more tender and flakier after grilling.
Whereas in the Kansai region (western Japan) the eel is slit down the belly and directly grilled without being steamed, often still in their original length, and called nagayaki (長焼き). The outer skin could be tough and chewy, and for this reason, eel cooked in Kansai style may be placed between layers of piping hot rice, in order for the vaporization to help tenderize it.
In the Kansai area, the eel is often called mamushi, just like the name of the common viper in Japan, Gloydius blomhoffii). Some speculate the name is a corruption of mabushi meaning "besprinkle', while others say it is a reference to the eel being rather similar to the viper in shape and vigor-endowing abilities when consumed.
There are several hypothesized origins for the name kabayaki. The name came to be generally written using the kanji 蒲焼 meaning cattail-broiled. Resemblance to the brown plush flower spikes of the cattail plant has been suggested as etymological origin in several old writings (Zokugo kō (『俗語考』 "Considerations on colloquial words" ); the writings of Mankō Morisada(ja); Kinsei jibutsu kō (『近世事物考』 "Considerations on near modern age items" )). Food historian Tekishū Motoyama (ja:本山荻舟)(1881–1958) has argued that originally the whole eel was brocheted vertically and cooked that way, giving rise to the name on the resemblance to the cattail both in form and color. This is incidentally the same as one hypothesized etymology for kamaboko.
Another touted theory explains the name as due to resemblance of the charred skin side to the kaba-kawa (樺皮, i.e. "beech-bark")  (Yōshū fushi (『雍州府志』 "Record of Yamashiro province" ), Honchō Seji Danki (『本朝世事談綺』 "Record of Yamashiro province" ). Motoyama also notes a proposed etymology from kōbashī (香ばしい, "fragrant (esp. of toasted or roasted items)").
- Heibonsha 1969, vol. 7,"kabayaki" by Tekishū Motoyama (ja:本山荻舟) describes it as being used principally or almost always for unagi (「もっぱら鰻」)
- Shinmura 1976 the Japanese dictionary says kabayaki applies to such fish as ungai, hamo, and dojō
- Hepburn 1888 J-E dict. defines "kabayaki: roasted flesh or fish".
- Prosek 2010, p.144-
- Bestor 2004, pp.167-8, repeats a story of a certain eel purveyor from Edo who asked a calligrapher to write the Ox (day) character as a sign
- Sasaki, Sanmi (佐々木三味); McCabe, Shaun; Iwasaki, Satoko (2005). Chado the Way of Tea: A Japanese Tea Master's Almanac. Tuttle Publishing. ISBN 9780804837163., p.372 describes Doyo as "about eighteen days from July 20th", though in actuality the beginning date can shift a day or two. The days cycle through the twelve animals (Twelve heavenly branches) so any one or two of the dates within the eighteen day period can fall on an Ox day.
- Bestor 2004, p.140
- PHP Kenkyusho (2007). 徹底比較江戸と上方(Tettei hikaku Edo to Kamigata). PHP研究所(PHP Kenkyusho). ISBN 9784569692791., p.9
- Barber, Kimiko; Martin Brigdale (photos) (2004). The Japanese Kitchen: A Book Of Essential Ingredients With 200 Authentic Recipes. Tokyo: Kyle Books. ISBN 9781904920021., p.144-
- Matsui 1984, p.9
- Nihon Dai Jiten Kankōkai (1972). 日本国語大辞典(Nihon kokugo dai jiten). Tokyo: Nihon Dai Jiten Kankōkai., p.67
- Motoyama, Tekishū(本山荻舟) (1958). 飲食事典 (Inshoku jiten). Heibonsha. p. 58.
- Hepburn, James Curtis (1888). A Japanese-English and English-Japanese Dictionary. Tokyo: Z.P. Maruya & Company. (4th edition)
- Shinmura, Izuru (1976). 広辞苑(Kōjien). Iwanami.
- Heibonsha (1969) . 世界百科事典(Sekai hyakka jiten). (world encyclopedia, in Japanese), vol. 7, article on "kabayaki" by Tekishū Motoyama (ja:本山荻舟) (1881-1958).
- Bestor, Theodore C. (2004). Tsukiji: The Fish Market at the Center of the World. University of California Press. ISBN 9780520220249.,p. 167-8
- Prosek, James (2010). Eels: An Exploration, from New Zealand to the Sargasso, of the World's Most Mysterious Fish. New York: Harper Collins. ISBN 9780060566111.
- Matsui, Isao (1984). Theory and Practice of Eel Culture. CRC Press. ISBN 9789061910367.