Source : WordNet®

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

Source : Free On-Line Dictionary of Computing

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 computer}s.  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 {VDU}s), 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)