If a figure is folded in half along a straight line and the two parts completely overlap, such a figure is called an axisymmetric figure, and this straight line is called an axis of symmetry. At this time, we also say that this figure is symmetrical about this line.

For example, isosceles triangle, square, equilateral triangle, isosceles trapezoid, circle and regular polygon are all axisymmetric figures. Some axisymmetric figures have more than one axis of symmetry. A circle has countless symmetry axes, and a straight line with the diameter of each circle is the symmetry axis of the circle.