Motion-to-photon latency
The time from a user moving their head to the right photons actually reaching the eye on the display — VR's single most important latency figure.
The chain spans sensor sampling, sensor fusion, game simulation, rendering, scanout to the panel and the pixel response itself. Let it add up to more than roughly 20 ms and the world starts to lag behind the head, and the brain answers with nausea. The figure became central as modern VR took off around 2013–2016, because everything else in a headset is subordinate to keeping this one number down.
The countermeasures attack every link: high refresh rates (90–120 Hz), sampling the head pose as late as possible before rendering, and reprojection such as timewarp, which rotates the finished image to an even fresher pose right before scanout. Foveated rendering frees up time by rendering the edge of the field of view coarsely.