Subset

In set theory, a subset of a set X is a set A whose elements are all elements of X: that is, $x\in A\Rightarrow x\in X$, denoted $A\subseteq X$. The empty set Ø and X itself are always subsets of X. The relation between the subset and the containing set is inclusion.

The power set of X is the set of all subsets of X.