A gender changer or "gender-bender", is a hardware device placed between two cable connectors of the same type and gender. An example is a cable connector shell with either two female or two male connectors on it (male-to-male or female-to-female), used to correct the mismatches that result when interconnecting two devices or cables with the same gender of connector.
Gender changers are used for RS-232C ports in either the original DB-25 or the IBM AT DE-9 format. They are also used when extending any sort of cable that normally has plugs on both ends (rather than a socket on one and plug on the other), however in this case it is usually called just an "extender", such as with F connectors, BNC connectors, and various RJ connectors used in telephony and computer networking.
The null modem is a computer communications adapter which may appear to be a gender changer, but it reroutes the wiring. The "transmit" pair from each side is routed into the "receive" pair of the other side, in the manner of a crossover cable.