The center of a triangle's circumcircle.

It is where the "perpendicular bisectors" (lines that are at right angles to the midpoint of each side) meet.

Try moving the points below, the circumcenter is where the lines meet:

It is where the "perpendicular bisectors" (lines that are at right angles to the midpoint of each side) meet.

Try moving the points below, the circumcenter is where the lines meet: