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

EIA-232
RS-232

    (Formerly "RS-232") The most common
   asynchronous serial line standard.  EIA-232 is the EIA
   equivalent of ITU-T V.24, and V.28.

   EIA-232 specifies the gender and pin use of connectors, but
   not their physical type.  RS-423 specifies the electrical
   signals.  25-way D-type connectors are common but often only
   three wires are connected - one ground (pin 7) and one for
   data in each direction.  The other pins are primarily related
   to hardware handshaking between sender and receiver and to
   carrier detection on modems, inoperative circuits, busy
   conditions etc.

   The standard classifies equipment as either Data
   Communications Equipment (DCE) or Data Terminal Equipment
   (DTE).  DTE receives data on pin 3 and transmits on pin 2
   (TD).  A DCE EIA-232 interface has a female connector.  DCE
   receives data from DTE on pin 2 (TD) and sends that data out
   the analog line.  Data received from the analog line is sent
   by the DCE on pin 3(RD).

   Originally DCE was a modem and DTE was a computer or terminal.
   The terminal or computer was connected (via EIA-232) to two
   modems, which were connected via a telephone line.

   The above arrangement allows a computer or terminal to be
   connected to a modem with a straight-through (2-2, 3-3) cable.
   It is common, however, to find equipment with the wrong sex
   connector or with pins two and three reversed, requiring the
   insertion of a cable or adaptor wired as a gender mender or
   null modem.  Such an adaptor is also required when
   connecting a computer directly to a terminal or to another
   computer without the use of modems.

   (1999-12-28)