Tempura is a typical Japanese dish usually consisting of seafood, meat and vegetables that have been battered and deep fried. The dish was introduced by the Portuguese in Nagasaki through fritter-cooking techniques in the 16th century. The word tempura comes from the Latin word tempora, a term referring to times of fasting when the church dictated that Catholics go meatless.
A light batter is made of iced water, eggs, and soft wheat flour (cake. Sometimes baking soda or baking powder is added to make the fritter light. Using sparkling water in the place of plain water makes a similar effect. Tempura batter is traditionally mixed in small batches using chopsticks for only a few seconds, leaving lumps in the mixture that, along with the cold batter temperature, result in the unique fluffy and crisp tempura structure when cooked. The batter is often kept cold by adding ice or placing the bowl inside a larger bowl with ice. Overmixing the batter will activate wheat gluten, which causes the flour mixture to become soft and dough-like when fried.
Specially formulated tempura flour is available in supermarkets. This is generally light (low-gluten) flour and occasionally contains leaveners such as baking powder.
Tempura does not use breadcrumbs (panko) in the coating. Deep-fried foods which are coated with breadcrumbs are called furai, Japanese-invented Western-style deep fried foods, such as tonkatsu or ebi furai (fried prawn).
No seasonings or salt is added to the batter, or the ingredients, except for some recipes recommending rinsing seafood in salt water before preparation.
Image and info from Wikipedia