Terminal emulator
A terminal emulator, terminal application, term, or tty for short, is a program that emulates a "dumb" video terminal within some other display architecture.
A terminal emulator inside a graphical user interface is often called a terminal window.
A terminal window usually supports a telnet, ssh, or dial-up shell session to a remote system that offers either a command line interface (CLI), such as a Unix shell, or to a character-cell application based.
The original character mode terminal applications typically provided a connection via modem to a BBS.
Example programs that provide this capability under Microsoft Windows include the built-in programs HyperTerminal and Microsoft's telnet client, 3rd party programs like PuTTY, SSH Secure Shell, and SecureCRT.
In the UNIX based environment, the CLI is the focus of operating system usage (however, with the introduction of graphical environments, it is not always so), and there are numerous programs for providing access to it from the X Window System GUI. The most mature of the bunch are xterm, dtterm, and rxvt.
