This site contains a comprehensive list of U.S. military terms and definitions.
XDM/X Model Experimental Development Model/Exploratory Development Model Xfer Transfer XIDB Extended Integrated Database XMIT Transmit XMTR Transmitter XO Executive Officer XOI USAFE Director of Intelligence XOIN USAFE Combat Operations Division XOIX USAFE Plans & Systems Division XP Director(ate) of Plans XRL X-Ray Laser XTR Transmitter XVIII ABC XVIII Airborne Corps