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