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

null modem

    A cable, especially an EIA-232
   cable, for connecting serial ports on two computers directly,
   rather than via modems.  Since, according to the
   specification, both computers should transmit on pin three of
   their EIA-232 connectors and receive on pin two, a null modem
   cable needs to connect one computer's pin two to the other's
   pin three and vice versa.  It also needs to have male
   connectors at both ends (again, according to the
   specification).

   (1996-05-17)