requested for a future time when it or they will be available to buy

"We have dozens of copies on back order", said the bookshop manager. 书店经理说：“我们有几十本书尚待补货。”

She said the masks are on back order and may not arrive for some time. 她说，这些口罩供不应求，可能要过一段时间才能到货。

We have lots more copies of the book on back order, so it's best to reserve a copy.

Fifty of the aircraft are in service, with over two thousand more on back order.

The manager told us that their coal stoves are on back order until June.

See also: back order noun