The collection of all the subsets of any set A is called the power set of A.

E.g., Consider the set A = {1, 2}. 

 Subsets of A = {1,2} are:  φ, { 1 }, { 2 } and { 1, 2 }.

Thus, the set of all these subsets is called the power set of A = { 1, 2 } and we write it as P( A ) = {φ,{ 1 }, { 2 }, { 1,2 }} ,  n [ P (A) ] = 4 =  .

In general, if A is a set with n (A) = m, then the total number of subsets =   and hence, n [ P(A) ] =  .

For more details, click on the link: https://www.youtube.com/watch?v=nv8MYeZgYI0&t=126s Timestamp: 36:00