An RMA (return merchandise authorization) 
is a numbered authorization provided by a mail-order merchant to permit the return 
of a product. Most mail-order businesses have a policy concerning returns. 
Some companies allow only defective products to be returned; others allow any software 
to be returned if it is unopened. 
To return a defective product, a typical process is:

First, the customer must call the technical support office and speak with a technician. 
The technician helps to determine whether the product is indeed defective. 
This usually involves the customer answering several questions and following the 
technician's directions for testing the product in question. If the technician feels that 
the product is defective, the technician issues an RMA. This process is especially important 
concerning software because most merchants do not allow the return of software unless the 
packaging is unopened.



Wikipedia says :-
A Return Merchandise Authorization or Return Material Authorization (RMA) is a transaction 
whereby the recipient of a product arranges to return defective goods to the supplier to 
have the product repaired or replaced or in order to receive a refund or credit for another 
product from the same retailer or corporation. In practice, an RMA is only issued after a series of tests.

The specifics vary between different retailers and industries, but in general the steps taken are similar.

Here are a couple of useful links about Return Merchandise Authorization (RMA) :-
What is an RMA ?
Wikipedia on RMA