The discriminant is a mathematical expression used in quadratic equations of the form ax^2 + bx + c = 0. It is calculated using the formula D = b^2 - 4ac. The value of the discriminant helps determine the nature of the roots of the equation. If the discriminant is positive, there are two distinct real roots. If it is zero, there is exactly one real root, also known as a repeated root. If the discriminant is negative, the equation has no real roots, but two complex roots. This information is crucial for understanding the solutions of quadratic equations.