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