Angle: Length of an arc relative to the circumference of the circle.

 A Visual or Demonstrational proof of a geometry fact.

The Fact: In a right angled triangle the area of the square on a leg is equal to the area of the rectangle formed by the hypotenuse and the projection of the leg on the hypotenuse.

Here is the proof of the above fact for the larger of the legs of a right angled triangle.