What is an "interference proceeding"?

This particular procedure is currently being phased out with the passage of the Leahy-Smith America Invents Act (AIA). Essentially, an interference proceeding is special proceeding (only available in the U.S.) which was meant to determine which party was first to invent some invention that was claimed by different patentees. Importantly, prior to the AIA, the United States was the only country in the world that was on a "first to invent" system. This meant that a first party that filed a patent application did not automatically win a priority contest against another party that could prove earlier conception and reasonable diligence.  

After the AIA was passed, the United States switched to a "first to file" system. As such, interference proceedings are no longer necessary. However, the old law still applies to patent applications with an effective filing date of March 16, 2013 or earlier.