A complex number is a number in the form $\displaystyle{a}+{b}{i}\usepackage{cmbright}\Huge\$ where a and b are real numbers. The term $\displaystyle{b}{i}\usepackage{cmbright}\Huge\$ is referred to as the imaginary part. So $\displaystyle\frac{{-{7}}}{{2}}{i}\usepackage{cmbright}\Huge\$ is the imaginary part.