Circle
A circle is the path of a point which must always be a given distance (the radius) from another point (the centre of the circle).
Explanation
The circle with radius r is given by the equation x2 + y2 = r2.
The number pi
You describe the area of a circle by A = πr2. Therefore, for the unit circle with radius r = 1, this area is the real constant π = 3,1415…. This is a transcendental number and thus has infinitely many decimal places. Yet the surface area is clearly bounded. This is just how nature works. There is only one word for it - incomprehensible.