Define Avogadro's constant. How is it related to a mole?

(N/A) Avogadro's constant $(N_A)$ is defined as the number of constituent particles (atoms,molecules,or ions) present in one mole of any substance. Its value is approximately $6.022 \times 10^{23} \text{ particles/mol}$.
Relationship to a mole: $A$ mole is a unit that represents a specific quantity of substance. By definition,one mole of any substance contains exactly $6.022 \times 10^{23}$ elementary entities. Therefore,the number of particles in one mole is equal to Avogadro's constant.