In order to understand what a terminal emulator is, you first need to know what the terms ‘terminal’, ‘mainframe’ and ‘GUI’ mean, because terminal emulation software was developed using GUI-based operating systems to ‘emulate’ the terminal’s role of  communicating with the mainframe.


Mainframes are large-scale computer systems that were the first form of computers. Referred to affectionately as ‘big iron’, they were once the size of rooms and were capable of high speed computing and the storage and handling huge amounts of data.

The development of the much smaller and more versatile PC led to the abandonment of the mainframe by many. However, those large organizations who had invested heavily in the technology of the time found it more cost-effective to continue using their mainframes, which also offer more reliability and data security than other systems.

However, the only means of communicating with the monolithic mainframe back then was via a terminal.


An IBM 3101 terminal
An IBM 3101 terminal.

If you picture one of those old televisions with the cathode ray tube and a green screen, that’s kind of what terminals were like. They were basic screens with a text-based input from a keyboard, through which an operator could communicate with the mainframe and receive outputs in response.

You had to be an expert to use one, they were cumbersome and limited in what they could do, and they had a limited life span as well. So when PCs arrived, many organizations recognised the advantages they offered and adopted the new technology.

Unfortunately, PCs and terminals were entirely incompatible so those organizations who also elected to continue with mainframes were also stuck with terminals. Eventually with advancements in the PC world terminal emulators became a reality allowing the terminal and the PC to combine.


A Graphic User Interface is a type of user interface that allows interaction with a computer through graphical means, such as windows, icons and menus. Developed first by Xerox and adopted by Microsoft and Apple for their PCs, it made communication with a computer much easier than text-based commands. So it was used by the developers of terminal emulation software to revolutionise communication with mainframes as well.

Terminal Emulators

So, in answer to the question, 'What is a terminal emulator?', it is a software program that mimics the same environment needed to access a mainframe, but through the GUI-based operating system of a modern PC rather than via an ancient text-based terminal.

And today, there’s terminal emulation software for every platform and it can be installed on any device from a tablet to a smartphone, allowing end users to access and communicate with their company mainframes from wherever they happen to be.