Partially Ordered Set
The following was adapted from Wolfram's site:
A partially ordered set (or poset) is a set taken together with a partial order on it. Formally, a partially ordered set is defined as an ordered pair $P=\left<X,\sqsubseteq \right>$, where $X$ is called the ground set of $P$ and $\sqsubseteq$ is the partial order of $P$. An element $u$ in a partially ordered set $\left< X,\sqsubseteq \right>$ is said to be an upper bound for a subset $S$ of $X$ if for every $s \in S$, we have $s \sqsubseteq u$. Similarly, a lower bound for a subset $S$ is an element $l$ such that for every $s \in S$, $l \sqsubseteq s$. If there is an upper bound and a lower bound for X, then the poset $\left< X,\sqsubseteq \right>$ is said to be bounded.
See PartialOrder