An abandoned cart email is an automated message sent to a shopper who added products to the cart but left without paying. It reminds the customer of the items left behind and invites them to complete the order, often with a clear call to action, product images and, when it makes commercial sense, a small incentive.
In eCommerce it is usually the highest-return flow in email marketing. A typical sequence has 2 to 3 emails over 24 to 72 hours, tested through A/B testing on timing, subject line and offer.