Subset
A subset of a set $X$, is a set $Y$ where each element of $Y$ is in $X$. In other words,
$$ Y \subseteq X \iff \forall y \in Y, y \in X. $$A subset of a set $X$, is a set $Y$ where each element of $Y$ is in $X$. In other words,
$$ Y \subseteq X \iff \forall y \in Y, y \in X. $$