A line which bisects a side of a triangle at right angle is called the perpendicular bisector of the side. Since a triangle has three sides, you can draw three perpendicular bisectors in a triangle.
The three perpendicular bisectors of the sides of a triangle pass through the same point, that is, they are concurrent. The point of concurrency O is called the circumcentre of the triangle.
If we take O as the centre and AO as the radius, you can draw a circle passing through the three vertices, A, B and C of the triangle, called Circumcircle of the triangle. O being the centre of this circle is called the circumcentre and AO the radius of the circumcircle is called circumradius of the triangle.
The circumcentre will be