The expression "-ax^2" represents a quadratic function where "a" is a positive constant. This function is characterized by a parabolic shape that opens downward due to the negative sign in front of "ax^2." The vertex of the parabola is the highest point, and the graph is symmetric about a vertical line called the axis of symmetry. In this context, "x" is the variable that can take on different values, affecting the output of the function. Quadratic functions like "-ax^2" are commonly studied in algebra and have applications in various fields, including physics and engineering, where they can model projectile motion and other phenomena.