This document defines key concepts related to quadratic functions and their graphs: 1) A quadratic function is a second-degree polynomial function of the form f(x) = ax^2 + bx + c, where a cannot be 0. 2) The graph of a quadratic function is called a parabola, which has a vertex and may open upward or downward depending on the sign of a. 3) The axis of symmetry is the vertical line through the vertex that divides the parabola into two equal parts.