U - radiation intensity 
U - Unavailability 
U - Underlined 
U - Unnumbered 
U - Usage 
u - user 
U - User Option 
U Frame - Unnumbered Frame 
U Plane - User Plane 
U-C - User Central 
U-N - User-to-Network 
U-NII - Unlicensed National Informatio… 
U-PCS - Unlicensed Personal Communicat… 
U-plane - User plane 
U-R - User Remote 
U-U - User-to-User 
U/C - Up-Converter 
U/D - Up/Down Chain 
U/L - Uplink 
U/S - User/Supervisor 
UA - Unassigned 
UA - Unavailability 
UA - Unbalanced ARM 
UA - Universal Access 
UA - Unnumbered Acknowledge 
UA - Unnumbered Acknowledgement 
UA - User Agent 
UAC - User Agent Client 
UAC - User Application Control 
UADSL - Universal Asymmetrical DSL