Domain theory is a concept in computer science that deals with the mathematical structures used to model the semantics of programming languages. It provides a framework for understanding how different types of data and computations can be represented and manipulated. By using partially ordered sets, domain theory helps in defining the behavior of programs, especially in the context of functional programming. In domain theory, each element in a domain represents a possible state of a computation, and the ordering reflects the idea of approximation. This allows for reasoning about convergence and continuity in computations, making it easier to analyze and optimize programs.