Define G in terms of the gravitational force between two masses. Write its value and SI unit.

Universal constant 'G' can be defined as the force of gravitational product between two bodies of unit mass each, separated by the unit distance between their centres. The SI unit of G is Nm^{2}kg^{-2}. Its value is 6.67*10^{-11}Nm^{2}kg^{-2}.