A coupon is a promotional tool that provides a discount or special offer on a product or service. For example, a coupon might say, "Get 20% off your purchase of $50 or more using code SAVE20." This type of coupon can be entered during online checkout or shown in-store. Coupons can offer free shipping, buy-one-get-one-free deals, or flat discounts, and are commonly distributed via email, printed ads, or coupon websites to encourage customer purchases.