X-PPAC stands for Extraterrestrial Phenomena Political Action Committee
X-E stands for X-Entertainment (website)
X-BOX stands for Microsoft console game system
X-KRSS stands for XML Key Registration Service Specification (W3C)
X(T)1 stands for Intermittent Exotropia at Near (walleye; ophthalmology)
X-COM stands for Extraterrestrial Combat Unit (game)
X(T) stands for Intermittent Exotropia at Distance (walleye; ophthalmology)
X(ALPHA)-RV stands for X(Alpha) Scattered-Wave Method
X-CRV stands for Experimental-Crew Return Vehicle (X-38 International Space Station Lifeboat; aka CRV, XCRV, X-38)
X-ALD stands for X-Linked Adrenoleukodystrophy (disease)