Return Merchandise Authorisation
or:
Return Material Authorisation (Aopen's version).

Appears to refer to the practice of sending back faulty computer hardware to the manufacturer. This TLA first noted on Ars Technica.

Example: "My bp6 is perfect. Now. Had to get it RMA'ed twice though."

An RMA is a tracking number issued by tech support after they have given up all hope and decided to allow you to return broken merchandise. It identifies your package, and allows the shipping department to accept only those returns with an RMA number. Any package without an RMA is assumed to be an unauthorized return and is refused.