Define conductivity and molar conductivity for the solution of an electrolyte. Why does the conductivity of solution decrease with dilution?

Conductivity of a solution at any given concentration is the conductance of 1 unit volume of solution kept between 2 electrodes which are unit length apart and have unit area of crossection. It is the conducting power of all ions confined in 1m³ of volume. Mathematically, it is the reciprocal of resistivity.

Molar conductivity is defined as conducting power of all ions produced by 1 mole of an electrolyte kept between the electrodes of a conductivity cell at unit distance but having area of crossection large enough to accommodate sufficient volume of the solution that contains 1 mole of electrolyte.

Conductivity of solution decreases with dilution because on dilution no. of ions in 1 unit volume decreases. As the no. of ions decreases the conductivity also decreases.