Self-aligning nut

A self-aligning nut, also known as a spherical nut or leveling nut, is a type of nut that is used in applications where the fastener is not perpendicular to the surface the nut anchors to. It achieves this action by using a flanged nut inside a specially shaped dished-out washer. They are commonly used in the aerospace industry. If this nut were not used the object would have to be spot faced perpendicular to the fastener.

...
Wikipedia