Marmalade is a fruit preserve, in English usage made of citrus fruits. British-style marmalade is sweet marmalade with a bitter tang made from fruit, sugar, water, and zest, which contains the bitter orange oil.
In English-speaking usage "marmalade" mostly refers to a preserve derived from a citrus fruit, commonly oranges. The recipe includes sliced or chopped fruit peel simmered in fruit juice and water until soft; indeed marmalade is sometimes described as jam with fruit peel (although manufacturers also produce peel-free marmalade).
Marmalade is often eaten on toast for breakfast. The favored citrus fruit for marmalade production in the UK is the "Seville orange", thus called because it was originally imported from Seville in Spain; it is higher in pectin than sweet oranges, and therefore gives a good set. Marmalade can be made from lemons, limes, grapefruits, oranges or any combination thereof.
*Processed in a plant that also processes wheat flour.