Pancake lenses
Short definition
Pancake lenses are a type of optics for virtual reality headsets that folds the light path between polarised layers, allowing thinner lenses placed closer to the display.
In plain language
By shortening the distance between the display and the eye, they make headsets more compact, with the weight closer to the face, which improves balance on the head. They usually give a sharp image across a larger area of the lens than Fresnel lenses. On the other hand, they let less light through, which requires brighter displays.
In practice, in an RVer session
RVer Headsets (Meta Quest 3) use pancake lenses.