Hi, There are different types of backing. AWS Standard Terms and Definitions defines backing as "a material or device placed against the back side of the joint adjacent to the joint root to support and shield molten weld metal. The material may be partially fused or remain unfused during welding and may be either metal or nonmetal. " Terms and Definitions defines "backing gas" as "backing in the form of a shielding gas employed primarily to provide a protective atmosphere." Example: ASME Section IX differentiates the two types of backing on their Suggested Format WPS form: The standard ASME Section IX form lists "backing" under the "joint" section (refering to metallic backing) and lists "backing" in the "shielding gas" section (refering to gas backing). What you have described is "inert gas backing", and yes it is a backing (in one of the two senses).
Best Regards,
Sol