A DTS sound format that simulates a surround sound experience using ordinary headphones.  The Headphone:X effect can come from the source device or be generated in the headphones themselves.  See 
DTS Surround Sensation and 
DTS.
A Headphone:X Gaming Headset
Using Headphone:X, the base station of Turtle Beach's Ear Force i60 converts the multi-channel audio from the PC into two-channel surround sound that transmits wireless to the headset.  The microphone is hidden in the i60.  (Image courtesy of Turtle Beach Corporation.)