fold

fold [fohld] verb

A common technique of combining mixtures (usually two) in a delicate manner. Folding is often employed when you want to retain air that has been beaten into egg whites or whipped cream, as when making soufflés or mousses. To combine, the aerated mixture is gently laid on top of the heavier mixture then the baker passes a rubber spatula through the layers across the bottom and “folds” the mixture up over the top until both are fully combined.  Recipes direct you to fold to keep the air bubbles in the mixture for a soft and light finished product. It is very distinct from stirring or beating.

Bakepedia Tips

Excessively rapid or heavy-handed folding will take air out of your mixture instead of retaining or adding it, so a light, gentle-but-quick rhythm will enable effective folding. We often begin this process (especially large amounts) with a balloon whisk, which naturally helps retain air in the mixture, and then finish with a rubber spatula.