Terminal emulation is the ability to make one computer terminal, typically a PC, appear to look like another, usually older type of terminal so that a user can access programs originally written to communicate with the other terminal type. Terminal emulation is often used to give PC users the ability to log on and get direct access to legacy programs in a mainframe operating system. Terminal emulation requires installing a special program in the PC or on a local area network (LAN) server to which it is connected. Typically, an enterprise with mainframe computers installs a terminal emulation program in all its workstations (or LAN servers). Workers can work locally with Windows or other PC or workstation applications and also open a window and work directly with mainframe applications. The terminal emulation program runs like any other workstation application as a separate program task providing its own window to the user. However, instead of content with a graphical user interface (GUI), the terminal emulation window presents some particular mainframe operating system or application interface that is text-only.
Different terminal emulation is required for specific types of terminals - for example, the IBM 3270 display terminal, the AS/400 5250 display terminal, or DEC's VT100 terminal. The program performing the terminal emulation must understand the data stream from the mainframe at several communication levels, including data link control and session control.