Definition: Custard is a culinary preparation made by blending eggs with milk or cream. Custard is thickened by the coagulation of the egg proteins, which is achieved by gently heating the custard in some way.
Custard is mainly used as a dessert, or as a base for a dessert, or as a dessert sauce.
Custard can be cooked in a bain-marie in the oven, or on the stovetop. Cooking custard in a bain-marie helps keep the cooking air moist and heats gently so that the custard doesn't curdle or crack.
A custard can have various consistencies, from thick and firm, as in creme brulee, to nearly liquid, as in creme anglaise. Pastry cream, or creme patissiere, which is used as a filling for classic desserts like cream puffs and éclairs, is made by adding some sort of starch, such as flour, to the custard.
Custard is usually sweet, but it can also be savory. Quiche is an example of a savory custard.