Jennie Schacht co-author of the Wine Lover's Dessert Cookbook defines Fold: "Fold is to mix together ingredients, generally lightly or delicately, without crushing or deflating them. To fold whipped egg whites into a batter, for example, gently whisk one-quarter to one-third of the whites into the heavier batter to lighten it, then scrape the remaining whites over the lightened batter. Combine the two by plunging a large rubber spatula into the center of the whites, going down to the bottom of the bowl and then scraping it up along the edge of the bowl and over the top to gently move the batter over the whites. Continue to work your way around, working rather swiftly (again to keep the whites from deflating) and turning the bowl each time, until the mixture is well integrated. With egg whites, you will generally want to leave them a bit streaky.  This technique is also useful for adding berries or other delicate ingredients into a batter, typically at the end."