By using Coulomb's law,define unit charge.

(N/A) In $SI$ units,the unit of charge is the Coulomb $(C)$.
According to Coulomb's law,the force between two point charges is given by $F = k \frac{q_1 q_2}{r^2}$,where $k = \frac{1}{4 \pi \epsilon_0} \approx 9 \times 10^9 \ N \cdot m^2/C^2$.
If we set $q_1 = q_2 = 1 \ C$ and $r = 1 \ m$,the force becomes $F = (9 \times 10^9) \times \frac{1 \times 1}{1^2} = 9 \times 10^9 \ N$.
Definition: $1 \ C$ is defined as the magnitude of charge that,when placed at a distance of $1 \ m$ from an identical charge in a vacuum,experiences an electrostatic repulsive force of $9 \times 10^9 \ N$.