Discriminant
The expression Δ = b² − 4ac that determines the nature of the roots of a quadratic equation.
The discriminant tells you, before you compute anything else, what kind of roots a quadratic equation ax² + bx + c = 0 has. A positive discriminant means two distinct real roots; zero means a single repeated real root; a negative discriminant means two complex conjugate roots.
Formula
Δ = b² − 4ac