Back order is a customer’s order that is unable to be filled.   In other words, the product is out of stock and the customer must wait on the product which they wanted to purchase.  The percentage of times backordered and the number of back order days are an important measure of the quality of company’s customer service and the effectiveness of the business’s inventory management. In some cases, back orders are not are not allowed.  This could result in lost sales when sufficient quantities are not available to completely ship an order.