Latency
Latency is the delay between action and reaction. Having low latency is crucial when using a Head-mounted Display for VR or AR. As you move your head, your HMD should display the resulting changes immediately. Any significant delay will cause your brain to reject the legitimacy of the virtual world and cause you to lose presence. High latency can also cause Motion Sickness.
Minimizing latency is one of VR and AR's biggest challenges. Human brains can detect very small latency in the visual and audio systems. According to John Carmack, these systems need to bring latency to below 20 millimeter to be undetectable by the brain.
Motion-to-photon latency / End-to-end latency
Motion-to-photon latency also known as the End-to-end latency is the delay between the movement of the user's head and the change of the display of VR device reflecting the user's movement. As soon as the user's head moves, the VR scenery should match the movement. The more delay (latency) between these 2 actions, the more unrealistic the VR world seems. To make the VR world realistic, VR systems want really low latency of 7ms to 15ms.
