Eye Relief
The distance between your eye and the rear scope lens at which you see the full image. Too close and the scope hits your face; too far and the image clips.
Most precision scopes have 3.5-4 inches of eye relief. The eyepiece-to-eye distance must stay in that band for the full field of view to appear.
Eye relief shifts slightly as magnification changes on a variable scope. Mount the scope so you find the sweet spot in your natural shooting position at the magnification you’ll use most, not at minimum or maximum.