How to Thin Client : Thin Client - Definitions

A thin client (sometimes also called a lean or slim client) is a client computer or client software in client-server architecture networks which depends primarily on the central server for processing activities, and mainly focuses on conveying input and output between the user and the remote server.

Many thin client devices run only web browsers or remote desktop software, meaning that all significant processing occurs on the server. However, recent devices marketed as thin clients can run complete operating systems such as Debian Linux, qualifying them as diskless nodes or hybrid clients. Some thin clients are also called "access terminals."

As a consequence, the term "thin client", in terms of hardware, has come to encompass any device marketed as, or used as, a thin client in the original definition – even if its actual capabilities are much greater.
The thin client is a PC with less of everything. The thin client is closely connected to the user interface.

In a thin client/server system, the only software that is installed on the thin client is the user interface, certain frequently used applications, and a networked operating system. By simplifying the load on the thin client, it can be a very small, low-powered device giving lower costs to purchase and to operate per seat. By keeping a few servers busy and many thin clients lightly loaded, users can expect easier system management and lower costs, as well as all the advantages of networked computing: central storage/backup and easier security.

From the user's perspective, the interaction with monitor, keyboard, and cursor changes little from using a thick client.
A single PC can usually power five or more thin clients. A more powerful PC or server can support up to a hundred thin clients at a time. A high-end server can power over 700 clients.

Users log onto the server using thin client hardware and the server creates a session in memory dedicated to that user. It is likely that the term "thin client" started to be used instead of "graphical terminal" for the following reasons:
When thin clients started to come back into vogue, fat clients had long been the norm in most environments. The term "thin client" is more descriptive and relevant than "graphical terminal", in an age in which all desktop computing devices have graphical capabilities.

Definitions

A thin client (or a lean client) is a network computer without a user writable long term storage device, which, in client/server applications, is designed to be especially small so that the bulk of the data processing occurs on the server. Thin client (computing): A server-centric computing model in which the application software, data, and CPU power resides on a network server rather than on the client computer.

Application program

A thin client does most of its processing on a central server with as little hardware and software as possible at the user's location, and as much as necessary at some centralized managed site.
Other definitions of thin versus thick/fat client application program try to draw the line at whether the deployment of the application requires the installation of additional software at the user site or not.

User-interface device

A thin client as a device is designed to provide just those functions which are useful for user-interface programs.

Executive Summary about Thin Client by Wikipedia