The graph of a quadratic function y = ax² + bx + c is called a parabola. The following graphs are two typical parabolas:

For the sake of explanation, we mark their x-intercepts by red dots, their y-intercepts by a pink dot, and the vertex of each parabola by a green dot:

The horizontal position of the parabola curve is determined by the linear function

 And this vertical line is called axis of symmetry.