In SAP, an instance refers to a separate runtime environment of the SAP system running on a single physical or virtual server. Each instance consists of one or more SAP system components, such as application servers, database servers, and message servers, configured to work together to support specific business processes and operations. Instances can be grouped into SAP system landscapes, which may include development, quality assurance, and production environments. Each instance has its own set of parameters and configurations, allowing for scalability, flexibility, and isolation of different system functions within the SAP ecosystem.