A Partially Ordered Set (or poset) is a mathematical structure consisting of a set of elements along with a binary relation that describes how these elements can be compared. In a poset, not all pairs of elements need to be comparable; that is, for some elements, neither can be said to be "less than" or "greater than" the other. The relation must be reflexive, antisymmetric, and transitive. An example of a poset is the set of subsets of a given set, ordered by inclusion. In this case, if one subset is included in another, it is considered "less than" the larger subset. This structure is useful in various fields, including mathematics, computer science, and order theory.