Field Curvature
An optical flaw where the focal surface is curved rather than flat, so the center and the edges of the image will not come into sharp focus together.
Field curvature occurs when a lens forms its sharpest image on a curved surface instead of a flat plane, which means the eye cannot focus the center and the edges at the same time. Set focus for a crisp center and the rim of the picture goes soft, or sharpen the rim and the middle blurs in turn. The effect is most noticeable at high magnification and toward the outer ring of the field of view.
Optical designers flatten the field with additional corrective elements, and a scope with good correction holds focus evenly from edge to edge. A modest amount of depth of field can mask small curvature, but it will not rescue a strongly curved design. Field curvature is separate from parallax, though both can leave a scope feeling soft if the shooter does not understand which one is at work.