Define electric current and explain it. Write its $SI$ unit.

(N/A) Electric current is defined as the net flow of charge through a cross-section of a conductor per unit time.
Consider a cross-section of a conductor where a net charge $q$ flows in time $t$. For a steady current,the current $I$ is given by $I = \frac{q}{t}$.
When the flow of charge changes with time,the instantaneous current $I$ is defined as the limit of the average current as the time interval approaches zero:
$I = \lim_{\Delta t \rightarrow 0} \frac{\Delta Q}{\Delta t} = \frac{dQ}{dt}$.
The direction of conventional current is taken to be the direction of motion of positive charge. Since positive charges do not flow in metallic conductors,the direction of current is taken opposite to the direction of motion of electrons.
The $SI$ unit of electric current is the Ampere $(A)$,where $1 \text{ Ampere} = 1 \text{ Coulomb/second}$.
One Ampere is defined as the current flowing through a cross-section of a conductor when $1 \text{ Coulomb}$ of charge flows through it in $1 \text{ second}$ ($6.25 \times 10^{18}$ electrons).