A poset, or partially ordered set, is a mathematical structure that consists of a set of elements along with a binary relation that describes how these elements are ordered. In a poset, not all pairs of elements need to be comparable, meaning that for some elements, neither can be said to be less than or greater than the other. The relation must be reflexive, antisymmetric, and transitive. Posets are commonly used in various fields, including computer science and mathematics, to model hierarchical structures. For example, the set of subsets of a given set can form a poset when ordered by inclusion. Other related concepts include lattices and Hasse diagrams, which visually represent the relationships within a poset.