Circles having the same centre but with different radii are said to be concentric circles. Two circles are said to be equal or congruent if they have equal radii.
A circle passing through all the vertices of a polygon is called a circumscribed circle of the polygon and its centre is called a circumcentre. The polygon is called an inscribed polygon.
A circle touching all the sides of a polygon is called an inscribed circle of the polygon and its centre is called incentre.