Government & Military

X&D Flot stands for Experiment and Development Flotilla (landing craft; US Navy)

X-CRV stands for Experimental-Crew Return Vehicle (X-38 International Space Station Lifeboat; aka CRV, XCRV, X-38)

X-POL stands for Cross-Polar

X-GDS stands for eXempt from the General Declassification Schedule

X-HOUR stands for Time crisis planning sequence is initiated by a Warning Order (precedes N-hour)

X-MIL stands for Extra Military Expense

X.509 stands for Standard for Public-Key Infrastructure