For the equation ax^(2) + bx + c (Quadratic equation) Which part is the discriminant?

The discriminant is that part of the quadratic equation under the square root symbol.

Specifically:

d = b² - 4ac

The sign of the discriminant tells you how many real roots there are.

d > 0 (positive) --> 2 real roots

d = 0 (zero) --> 1 real (double) root

d < 0 (negative) --> 0 real roots (and 2 complex roots)

That's an expression/polynomial, not an equation, but the discriminant is b^2 - 4ac