Eye Relief
The distance between your eye and the rear lens of the scope at which you see the full image. Too close and the scope hits your face under recoil; 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.