Batter is a liquid mixture, usually based on one or more flours combined with liquids such as water, milk or beer. Egg is also a common component. Often a leavening agent is included in the mixture to aerate and fluff up the batter as it cooks (or the mixture may be naturally fermented for this purpose as well as to add flavour).
The viscosity of batter may range from very "stiff" (adhering to an upturned spoon) to very "thin" (similar to single cream, thin enough to pour or drop from a spoon and sometimes called "drop batter”). Heat is applied to the batter, usually by frying, baking or steaming, in order to cook the ingredients (thus rendering them palatable) and to "set" the batter into a solid form. Batters may be sweet or savoury, often with either sugar or salt being added (sometimes both). Many other flavourings such as herbs, spices, fruits and vegetables may also be added to the mixture.
The word 'batter' comes from the old French word 'battre' which means 'to beat', as many batters require vigorous beating or whisking in their preparation.
Batters may be used to make crepes, dosais, idlis, vadais, Yorkshire pudding, fritters, doughnuts, hushpuppies, tempura, cakes, some breads, to coat meat or vegetables when frying, or to make pancakes, waffles or some cookies.
Batter is also very commonly used in some parts of the world to coat fish before frying. Batter makes up a critical component of the famous British dish Fish and chips for instance.