To fold, in culinary terms, means to combine a light and airy ingredient, like beaten egg whites, with a heavier ingredient, like a custard or cream. The technique involves placing the heavy ingredient in the bottom of a bowl, with the lighter one on top of it. A spatula is then used to cut the mixture across the bottom of the bowl, starting from one side, and continually rotating a quarter of a turn, eventually combining both ingredients by turning the ingredients on top of each other.