Astigmatism
An aberration in which lines running radially and tangentially from the frame centre focus at different distances.
In depth
An astigmatic lens can never render both sets of lines sharp at once off-axis, so as focus is shifted the corners appear to smear first one way and then the other. It is why lens tests report separate sagittal and meridional MTF curves: the gap between those two lines is a direct measure of astigmatism.
Examples in practice
A brick wall shot at the corners shows the vertical mortar lines sharp while the horizontal ones are soft, then the reverse a fraction of a turn later.