Short version: Checkout is the final order process where a customer completes delivery, billing, payment, and confirmation steps.
In an online store, checkout is where buying intent either becomes an order or turns into friction. The customer may need to enter contact details, delivery address, shipping method, billing information, payment method, discount code, and final consent before placing the order.
Good checkout design is clear, trustworthy, and short enough to finish without confusion. It should show costs up front, support the payment methods customers expect, handle errors politely, avoid unnecessary account creation, and make the final action unambiguous. Even small problems here can affect revenue because the customer is already close to conversion.
Related terms: cart, checkout flow, and conversion rate.