Customers who have stopped buying from the store a while ago. For one thing, these customers are either satisfied with their one-time purchase such as Christmas & Gift Buyers or simply unsatisfied. They can be considered inactive as they have been gone for a long time without any orders.
To turn them in again, it would be interesting to see what channels these customers were originally acquired through, as they may represent an under-performing channel. It would be worth to identify unsatisfied customers to check what went wrong.