1) In data communications, a terminal is any device that terminates one end (sender or receiver) of a communicated signal. In practice, it is usually applied only to the extended end points in a network, not central or intermediate devices. In this usage, if you can send signals to it, it's a terminal.
3) In computers, a terminal (sometimes qualified as a "dumb" terminal) is an end-use device (usually with display monitor and keyboard) with little or no software of its own that relies on a mainframe or another computer (such as a PC server) for its "intelligence." IBM's 3270 Information Display System was a widely-installed system of such terminals in corporations. Many applications designed for the 3270 or other "dumb" terminals are still in use at PCs that emulate or act like a 3270. The VT-100 from Digital Equipment Corporation is another example of a widely-used so-called "dumb" terminal. A variation of this kind of terminal is being revived in the idea of the thin client or network computer.
4) The term is sometimes used to mean any personal computer or user workstation that is hooked up to a network.