The user-facing part of an application. In a desktop application this would include the windowing framework and the forms the user interacts with; in a command line program it would be the available commands and arguments; and in a web app it would refer to the HTML and JavaScript.
In software engineering, the terms front end and back end refer to the separation of concerns between the presentation layer (front end), and the data access layer (back end) of a piece of software, or the physical infrastructure or hardware. In the client–server model, the client is usually considered the front end and the server is usually considered the back end, even when some presentation work is actually done on the server itself.
