Eyebox
The three-dimensional zone behind the scope where the shooter's eye can sit and still see a full, sharp image.
Eyebox is the combination of eye relief tolerance and lateral forgiveness. Premium scopes (Tangent Theta, Nightforce ATACR, ZCO) have famously generous eyeboxes. You can find the picture from a slightly imperfect cheek weld.
Cheap scopes often have tight eyeboxes that turn into edge-darkening or full blackout if your eye drifts a few millimeters off-axis. This costs more than people realize: every shot taken from a forced head position is a shot with degraded sight picture.