Stir in is a cooking term that means to mix an ingredient into a dish by gently folding it in with a spoon or spatula. It is often used when adding delicate ingredients such as eggs, cream, or melted butter to a mixture. Stirring in helps to evenly distribute the ingredient throughout the dish and prevents it from clumping together. Stirring in is also used to incorporate air into a mixture, which can help to create a light and fluffy texture.