An axis of symmetry is the line that divides the graph of a function into two mirrored images. Consider the graph of a quadratic function which is a parabola that has an axis of symmetry that is parallel to the y-axis and passes through the vertex. It is also possible for a function to have an axis of symmetry which is not vertical. Take the following graph as an example.

What is more, if a function's axis of symmetry is the y-axis, 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 a line exists that divides the figure into congruent, mirror-image halves.