*Regular Polygon
A polygon which is both equiangular and equilateral (i.e. having all sides the same length and all interior angles the same).
Note that this doesn't preclude the existence of a square with curved sides if projected upon a sphere. But when discussing common geometry the assumption is that we are working within a single plane. If you wish to work in non-planar geometry, that must be explicitly called out in the diagram.
It's weird to picture, but every point on the circle has a tangent vector which is, by definition, at a right angle to the radius vector, which is what the sides of that larger pie piece both are.
You have to think of the infinitesimally small point where the two meet.