Glossary
Eye relief
The distance between the eye and the optics at which the whole of a display's image can still be seen.
Eye relief is the distance from the eye to the optics within which the full image remains visible. Move beyond it and the edges of the image are lost before the centre is.
Why it appears on a glasses spec sheet
It decides whether a display works on your face rather than on a mannequin. Generous eye relief tolerates a frame that sits further from the eye, a different nose bridge, or a prescription lens between eye and optics. Tight eye relief means the display is only fully visible when the frame sits just so, and that is a fit question no specification sheet can answer for an individual.