What's the meaning of backorder? I tried looking it up in the dictionary as well, but couldn't find it. Could someone help?

for eg. Books are on backorder.
Hello Sonia,

Firstly, I will tell you my guess of what backorder means.

I think that backorder means that the company does not have the book in stock at this time, so they can't ship it to you right away. This is because they do not have enough room to keep all books in stock all the time. When they get enough orders for the out of stock book from customers, they place an order to get it. Later, when they have it in stock, they send it to you. So, when you place an order for something that has backorder status you will not get it as quickly as when you order a book that is already "in stock".

Secondly, I will search google.com to see what they say.
Ok. I made this search, define: backorder, and got this result:

Definitions of backorder on the Web:

A current or past due customer order (or line item) that cannot be shipped due to lack of inventory availability. Customer agreements govern how backorders are handled (ship when available, ship whole order only, cancel, etc.).

—an order that is past its required date for an item that has inadequate inventory to fill the order.

The delay of a line item or order as a result of a stockout.

An order to be filled when stock is renewed.

An order placed to fill a definite order from a dispenser. Also called short order, or special order

I showed you the google.com define trick
so you can try it yourself
next time you need a word defined.

Emotion: geeked

Teachers: We supply a list of EFL job vacancies
Trellis, you are wonderful! That is a really helpful trick. That is specially for you. (f)
Hi Maj,

Thanks for the rose. Emotion: smile

Only a few days ago I learned the define: any-word-here
search trick that you can do at google.com and I like it a lot.

It is very handy.
I am glad you like it too.

(you forgot the space after the flower icon, so i fixed it for you)