An Application is used to represent an operational IT system, which is providing functions and services needed by  business users to undertake their processes.

For example, Enterprise Resource Planning system, as well as their components such as Supply Chain Management, Inventory Management, Finance and Human Resource Management system would all be considered to be Applications.

Applications are typically used in various Architectural Artefacts, such as a Baseline  / Current State and Target / End State Application Architecture Diagrams.