The answer to the essence and definition of axial symmetry is as follows:

If a graph is folded in half along a straight line, the graphs on both sides of the straight line can completely overlap, and this graph is an axisymmetric graph. The straight line where the crease lies is called the symmetry axis.

Its properties are as follows: if two figures are symmetrical about a straight line, then the symmetry axis is the middle perpendicular of the line segment connected by any pair of corresponding points; The symmetry axis of an axisymmetric figure is the perpendicular bisector of a line segment connected by any pair of corresponding points.

Generally speaking, if all points on two graphs are symmetrical about the same straight line, they are said to be symmetrical about this straight line. This symmetry about a straight line is called axial symmetry, and this straight line is called the axis of symmetry of mutually symmetrical figures.

In 2008, page 29 of the eighth grade textbook of People's Education Press defined the symmetry axis in more detail: if a plane figure is folded along a straight line, the parts on both sides of the straight line can overlap each other, this figure is called a symmetrical figure, and this straight line is its symmetry axis.

Fold a graph along a straight line. If it can overlap with another graph, then the two graphs are said to be symmetrical about this straight line, which is called the axis of symmetry.

That is to say, both axisymmetric graphics and axisymmetric graphics have axes of symmetry. If an axisymmetric figure is divided into two parts along the axis of symmetry, then the two figures are symmetrical about this line; If two symmetrical figures are regarded as a whole, then it is an axisymmetric figure.

The exact definition of symmetry axis generally does not appear in primary school mathematics textbooks. But through real life, students are guided to get a preliminary understanding of symmetrical figures, which leads to the concept of "symmetry". Then students can cut out symmetrical figures by hand and understand that the crease of paper is the symmetry axis.