The discriminant is the number $\displaystyle{{b}}^{{2}}-{4}{a}{c}\usepackage{cmbright}\Huge\$ , and the solutions to a quadratic equation are given by the formula:

$\displaystyle{x}=\frac{{-{b}\pm\sqrt{{{{b}}^{{2}}-{4}{a}{c}}}}}{{{2}{a}}}\usepackage{cmbright}\Huge\$

If $\displaystyle{{b}}^{{2}}-{4}{a}{c}={0}\usepackage{cmbright}\Huge\$ , then the radical would be 0 and there would be one real solution.