Subset
From AoPSWiki
We say a set 
is a subset of another set 
if every element of is also an element of , and we denote this by 
. The empty set is a subset of every set, and every set is a subset of itself. The notation 
emphasizes that may be equal to , while 
says that is any subset of other than itself.
The following is a true statement:
The set of all subsets of a given set 
is called the power set of and is denoted 
or 
.
