A regular octagon possesses a total of $8$ lines of symmetry, which can be identified by drawing lines through its vertices and the midpoints of its sides.

To understand the lines of symmetry in a regular octagon, we must recognize that these lines divide the shape into two identical halves. Given that a regular octagon has equal sides and angles, there are two types of symmetry lines: those that connect opposite vertices and those that connect the midpoints of opposite sides.

Begin by sketching the octagon and labeling its vertices from $A$ to $H$. The first set of symmetry lines can be created by connecting each vertex to its directly opposite vertex. For instance, draw a line from vertex $A$ to vertex $E$, from vertex $B$ to vertex $F$, and continue this process for the remaining vertices. This method yields $4$ lines of symmetry.

Next, locate the midpoints of each side of the octagon. Draw lines connecting the midpoints of opposite sides. For example, for the side connecting vertices $A$ and $B$, find the midpoint and draw a line to the midpoint of the side that is directly opposite, which is between vertices $E$ and $F$. Repeat this process for all pairs of opposite sides, resulting in an additional $4$ lines of symmetry.

By combining these two methods, we can confirm that a regular octagon has a total of $8$ lines of symmetry, each of which divides the shape into two mirror-image halves. This symmetrical property is a fundamental characteristic of regular polygons and is essential for understanding their geometric properties.