Cross-selling is a marketing and sales technique in which a seller suggests or offers related or complementary products or services to a customer who is already making a purchase. The idea is to encourage the customer to buy additional products or services that are related or complementary to the one they are already buying.

For example, a customer who is buying a laptop may be offered an additional battery, a protective case, or a software upgrade as a cross-sell item. Similarly, a customer who is buying a camera may be offered additional lenses, a memory card, or a tripod.

Cross-selling can be beneficial for both the customer and the seller. Customers can benefit by discovering and purchasing additional products that complement the one they are buying, while sellers can benefit from increased sales and revenue from cross-sell items.

Cross-selling is often used in online retail and e-commerce, where algorithms and personalized recommendations are used to suggest related products based on the customer's browsing and purchase history.