The Free On-line Dictionary of Computing (30 December 2018):

IBM 3270
3270

    A class of terminals made by IBM known as
   "Display Devices", normally used to talk to IBM
   mainframes.  The 3270 attempts to minimise the number of
   I/O interrupts required by accepting large blocks of data,
   known as datastreams, in which both text and control (or
   formatting functions) are interspersed allowing an entire
   screen to be "painted" as a single output operation.  The
   concept of "formatting" in these devices allows the screen to
   be divided into clusters of contiguous character cells for
   which numerous attributes (color, highlighting, character
   set, protection from modification) can be set.  Further,
   using a technique known as 'Read Modified' the changes from
   any number of formatted fields that have been modified can be
   read as a single input without transferring any other data,
   another technique to enhance the terminal throughput of the
   CPU.

   The 3270 had twelve, and later twenty-four, special Programmed
   Function Keys, or PF keys.  When one of these keys was
   pressed, it would cause the device to generate an I/O
   interrupt and present a special code identifying which key
   was pressed.  Application program functions such as
   termination, page-up, page-down or help could be invoked by a
   single key-push, thereby reducing the load on very busy
   processors.

   A version of the IBM PC called the "3270 PC" was released in
   October 1983.  It included 3270 terminal emulation.

   tn3270 is modified version of Telnet which acts as a 3270
   terminal emulator and can be used to connect to an IBM
   computer over a network.

   See also broken arrow.

   (1995-02-07)