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; 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. Posets are commonly used in various fields, including computer science, mathematics, and logic. They help in organizing data, representing hierarchies, and solving problems related to ordering and ranking. Examples of posets include subsets of a set ordered by inclusion and the set of natural numbers ordered by the usual less-than relation.