• a circle that passes through all the vertices of a polygon.
• the centre of a circumcircle is called the circumcentre
and the radius is called the circumradius.
• all regular polygons, triangles, rectangles and the
isosceles trapezium (UK)/trapezoid (US)
may be circumscribed with a circle.
• Other irregular polygons are not usually considered to have a circumcircle.
However certain irregular shapes may qualify.