3270 terminal

The IBM 3270 was a family of display terminals introduced in 1971. The physical hardware is long gone from most sites, but the protocol it spoke is still how people connect to z/OS, now through software emulators running on ordinary computers.

Its defining feature is that it is block-mode. Instead of sending each keystroke as you type, the terminal holds the whole screen locally and only transmits when you press Enter or a function key. That design existed to reduce network traffic, and it is why mainframe screens feel different from anything else: you fill in the form, then send it.

A 3270 screen is a grid of character positions, with some fields protected (labels you cannot type over) and some unprotected, where your input goes. Colour and highlighting are limited and used deliberately, often to mark errors.

Anything that presents you a mainframe screen, whether a desktop emulator or a browser-based one, is speaking 3270 underneath. The name comes up constantly in documentation.