An axis of symmetry is the line that divides the graph of a function in two mirrored images. For example, the graph of a quadratic function is a parabola that has an axis of symmetry parallel to the $y\text{-}$axis and passes through its vertex. It is also possible for a function to have an axis of symmetry which is not vertical. The following graph shows an example of this.

If a function's axis of symmetry is the $y\text{-}$axis, it is said that it is an even function.

The concept of an axis of symmetry extends beyond graphs of functions. Any geometric figure can have an axis of symmetry if there exists a line that divides it into congruent, mirror-image halves.