A "closed-domain" refers to a specific area of knowledge or a limited set of topics where information is well-defined and contained. In this context, systems or applications are designed to operate within these boundaries, making them efficient for tasks like answering questions or providing information related to that domain. Examples include customer support systems for a particular product or medical diagnosis tools focused on specific diseases. In contrast to open-domain systems, which handle a wide range of topics, closed-domain systems are tailored for particular needs. This specialization allows for more accurate responses and better performance in their designated areas, such as chatbots for banking or healthcare applications.