Pieces of raw or nearly-raw fish or other seafood served most often in Japanese restaurants, usually presented in an artistic fashion and using only the highest quality fish.
Thickness: Most sashimi should not be sliced paper-thin, but rather thick enough that the individual type and cut of fish manifests its own distinctive texture. Second-rate or previously-frozen fish is often sliced very thin to disguise an inferior texture.
Raw: Not all sashimi is served raw. In particular, a number of fish are generally lightly salt-pickled, the most common of these being mackerel (saba). Some seafood may be served raw or cooked, notably many crustaceans. Some meatier fish, such as bonito, may be seared on the outside and sliced, a preparation known as tataki.
Soy and Wasabi: The most common dipping sauce for sashimi is certainly this combination, but the apparent simplicity covers significant variability. Japanese soy sauce (shoyu) --- do not use Chinese soy sauce, which has a quite different flavor --- comes in a range of styles and strengths, and these may be selected regionally, seasonally, and/or to complement particular fish. Wasabi is an uncommon and expensive root unrelated to horseradish, despite the taste similarity. The majority of wasabi, in Japan and especially elsewhere, is actually green-dyed horseradish, and lacks the subtle floral flavor of the real wasabi. Purists who insist that one should never mix wasabi into soy sauce to make a paste should be referring exclusively to real wasabi in combination with excellent, small-production soy sauce; otherwise, it makes very little difference, if any, how it is mixed.
Ponzu: An excellent alternative, particularly in hot weather, ponzu (ポン酢) is a combination of soy sauce, citrus juice, and sake, often enhanced in flavor through the use of kombu seaweed and dried bonito flakes (katsuo-bushi).
Tosazu: A sauce originally from the old province of Tosa (now Kōchi), in southern Shikoku. It is very good with oilier, stronger-flavored fish, such as bonito (katsuo), for which this province is famous (especially as katsuo-tataki). Tosazu (土佐酢) is made from soy sauce, vinegar and/or sake, deepened with dashi and/or kombu and katsuo-bushi; it may also be aged before use.
In the Tokyo region (Kantō), the dish is known as sashimi or osashimi (お刺身). In Kyoto, and intermittently throughout the Kansai region, it is called otsukuri (お造り)