Definition: gently combining a light mixture with a heavier one using a cutting-and-turning motion, to keep air in.
Folding is how you add something delicate — whisked egg whites, whipped cream — to a heavier batter without knocking the air back out. Use a large spoon or spatula: cut down through the middle, sweep along the bottom, and turn the mixture up and over, rotating the bowl as you go. Slow and deliberate beats fast and thorough; a few streaks left is better than a deflated, over-mixed batter.
In practice: stop as soon as it’s just combined — every extra stroke costs you air.
Related:creaming,French meringue.