What do you understand by the term current ? State and define its S.I.unit.

Current is the rate of flow of charge across a cross-section. It is a scalar quantity.
Its S.I. unit is ampere (coulomb per second).
If 1 ampere current flows through a conductor, it means that 6.25 x 10¹⁸ electrons pass in 1 second across that cross-section of conductor.