In the set theory branch of mathematics,
The set A is said to be a subset of B if and only if every element on A is also an element of B.
or
The set A is a subset of the set B (denoted: A ⊆ B) iff
∀x ( x∈A → x∈B )
(In English, that's "for all x, 'x is an element of A' implies 'x is an element of B'")