Terminal emulation is the ability of a personal computer (which is programmable and therefore "smart") to appear to be a "dumb" (nonprogrammable) terminal" so that it can be used to interact with a mainframe computer or other computer with its own proprietary connection interface. With terminal emulation, a PC user can log on and get direct access to programs in the 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 terminal - for example, the IBM 3270 display terminal or the AS/400's 5250 display 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. Special hardware may be required, depending on how the PC using the terminal emulation program is being connected to the mainframe - for example, through remote dial-up, leased line, or ISDN.

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