Discriminant of a polynomial

In algebra, the discriminant of a polynomial is an invariant which determines whether or not the polynomial has repeated roots.

The discriminant may be defined as the resultant of the polynomial and its derivative.

Examples
The discriminant of a quadratic $$aX^2 + bX + c$$ is $$b^2 - 4ac$$, which plays a key part in the solution of the quadratic equation.