Drift Velocity—

The drift velocity is defined as the average velocity gained by the free electrons of a conductor in the opposite direction of the externally applied electric field.

Relaxation Time—

The average time that elapses between two successive collisions of an electron is called relaxation time.

Resistivity—

The  resistance R of a conductor is

The resistivity or specific resistance of a material may be defined as the resistance of a conductor of that material, having unit length and unit area of cross section.

Ohm’s Law—

Ohm’s law states that the current flowing through a conductor is directly proportional to the potential difference applied across its ends, provided the temperature and other physical conditions remain uncharged.

V=IR

Where, R is proportionality constant and known as Resistance of The Conductor.

Electric Current—

Electric Current in a conductor across an area held perpendicular to the direction of flow of charge is defined as the amount of charge flowing across that area per unit time.  

Its unit is ampere or Coulomb/sec.