Retrofit is a term used to describe the process of adding new parts or components to an existing system or structure. It is often used in the context of technology, where it refers to the process of updating older systems with newer parts or components. Retrofitting can also be used in other contexts, such as architecture, where it refers to the process of adding new features or components to an existing building. In all cases, retrofitting involves making changes to an existing system or structure in order to improve its performance or functionality.