Custard is a family of preparations based on milk and eggs, thickened with heat. Most commonly, it refers to a dessert or dessert sauce, but custard bases are also used for quiches and other savoury foods.

As a dessert, it is made from a combination of milk or cream, egg yolks, sugar, and flavourings such as vanilla. Sometimes flour, corn starch, or gelatin are also added. In French cookery, custard—confusingly called just crème—is never thickened in this way: when starch is added, it is pastry cream crème pâtissière; when gelatin is added, it is crème anglaise collée.

Depending on how much egg or thickener is used, custard may vary in consistency from a thin pouring sauce (crème anglaise), to a thick blancmange like that used for vanilla slice or the pastry cream used to fill éclairs.

Custard thickened with starch is a non-Newtonian fluid which in short means that if impacted with sufficient force it behaves more like a solid than a liquid, as a consequence, as was dramatically demonstrated on Sky Television's Brainiac: Science Abuse programme, it is possible for a full-grown adult to walk across a a swimming pool filled with custard, without sinking.

Most custard is cooked in a double boiler (bain-marie) or heated very gently on the stove in a saucepan, but custard can also be steamed or baked in the oven with or without a hot water bath.

Custard is an important part of dessert recipes from many countries, including the United Kingdom, France, Portugal, Spain, Italy, and Australia.

Instant and ready-made 'custards' are also marketed, though they are not true custards if they are not thickened with egg. See Bird's Custard, for instance.

http://en.wikipedia.org/wiki/Custard