What is lines of symmetry?

Answer 1

A line of symmetry is a line through a shape. It shows that on either side of this line that the picture is exactly the same. There can be more than one line of symmetry in a shape.
They are 'mirror images' when a shapes are divided by a lines of symmetry.

Answer 2

For a 2-dimensional object, a line of symmetry is a line such that, if you were to fold the object along that line, the two sides would match up exactly.

For a 3-dimensional object, a line (axis) of symmetry is a line such that, if you were to rotate the object around it, its shape would remain the same.

Answer 3

When if you split something in half, it will look like one of the sides is looking into a mirror.