Congress of Industrial Organizations n (Government, Politics & Diplomacy) (in the US) a federation of industrial unions formed in 1935. It united with the AFL in 1955 to form the AFL-CIO. Abbreviation: CIO Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014