WordNet (r) 3.0 (2006):

serial port
    n 1: an interface (commonly used for modems and mice and some
         printers) that transmits data a bit at a time

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

serial port

    (Or "com port") A connector on a
   computer to which you can attach a serial line connected to
   peripherals which communicate using a serial (bit-stream)
   protocol.  The most common type of serial port is a 25-pin
   D-type connector carrying EIA-232 signals.  Smaller
   connectors (e.g. 9-pin D-type) carrying a subset of EIA-232
   are often used on personal computers.  The serial port is
   usually connected to an integrated circuit called a UART
   which handles the conversion between serial and parallel data.

   In the days before bit-mapped displays, and today on
   multi-user systems, the serial port was used to connect one
   or more terminals (teletypewriters or VDUs), printers,
   modems and other serial peripherals.  Two computers
   connected together via their serial ports, possibly via
   modems, can communicate using a protocol such as UUCP or
   CU or SLIP.

   (1995-01-12)