Bird's Custard (a brand name) is the original version of what is known generically as custard powder. It is a cornflour-based powder which thickens to form a custard-like sauce when mixed with milk and heated to a sufficient temperature. Bird's Custard was first formulated and first cooked by Alfred Bird in 1837, because his wife was allergic to eggs, the key ingredient used to thicken traditional custard.
In some regions, such as some parts of the United Kingdom, the popularity of this type of dessert is such that it is simply known as "custard." In such cases, general usage of the word may be more likely to refer to the "Bird's" custard rather than to the traditional egg-based variety.
In recent years, "instant" versions (containing powdered milk and sugar and requiring only hot water) and ready-made custard in tins and cartons have also become popular.