Circumcircle is the smallest circle passing through each of the corner points of a regular polygon. The radius of the circumcircle is equal to the radius of the regular polygon. It is called the Circumradius.
The center of the Circumcircle is the intersection of the diagonals of the regular polygon called Circumcenter. It coincides with the center of the regular polygon
The smallest circle possible through the four vertices of the square is called the Circumcircle of a Square. The center of the Circumcircle is the intersection point of the two diagonals of the square, called Circumcenter of a Square.