Fat and thin client architecture

Fat client
A fat client (sometimes called heavy) is a client in client–server architecture that typically provides rich functionality independent of the central server.

A fat client still requires at least periodic connection to a network or central server, but is often characterised by the ability to perform many functions without that connection. In contrast, a thin client generally does as little processing as possible and relies on accessing the server each time input data needs to be processed or validated.

Thin client
A thin client (sometimes also called a lean or slim client) is a computer or a computer program which depends heavily on some other computer (its server) to fulfill its traditional computational roles.

The most common type of modern thin client is a low-end computer terminal which concentrates solely on providing a graphical user interface to the end-user. The remaining functionality, in particular the operating system, is provided by the server.

In web development in particular, client applications are becoming fatter. This is due to the adoption of heavily client-side technologies like Ajax and Flash, which are themselves strongly driven by the highly interactive nature of Web 2.0 applications.

With thin client architecture, the server, in taking on the whole processing load of several clients, forms a single point of failure for those clients. This has both positive and negative aspects.