In computing, a Dumb Terminal (sometimes called Thin Client), refers to a computer that has no independent processing power, but instead relies on the power of other computer(s) or resources that it is connected to via a network. Dumb Terminals are most commonly found in large organizations with the need to process immense numbers of data transactions, such as financial institutions. They generally are capable of inputting and receiving information from a database or network resource that it is connected to, but otherwise have no processing power or capabilities. 
The earliest dumb terminals were mechanical Teletypes, and then "glass teletypes" that emulated them on a television-like screen, with much reduction of size and noise. As user requirements for display and interaction became more and more complex, such as, for the financial transactions required, having "protected fields" (e.g., "name", "account number") separate from "data entry fields" made it more and more complex to build the appropriate terminal with pure hardware.
An interim step was to go to dumb terminals on "cluster controllers". In these configurations, several local dumb terminals, as in a bank branch, cabled locally to a "cluster controller" that connected to the network. The cluster controller held the more complex hardware, shared among the devices. As microprocessors of low cost became available, they first moved into the cluster controller, and then to the terminals themselves -- the cluster controller began to disappear. Even apparent "dumb terminals" made today contain dedicated microcomputers, simply because that is the cheapest way to build them.
The use of Dumb Terminals on corporate mainframes was common in the 1970's and 80's, but since then has largely been replaced by terminal emulation and virtualization.  Telnet remains the main standard, still in use, for dumb terminal emulation, also called "network virtual terminal". Teknet protocols have been in use since 1972, and the specifications are spread across a wide range of documents. The first, however, is RFC 318.