Dear Friends,
Perhaps this will help clear up the confusion.
A "diplexor" is a passive device that lets you split and/or combine signals based on their frequencies. It does not matter whether the frequencies are close together or on different bands nor does it matter if you are using it for transmitting or receiving.
A diplexor is commonly used in radio and TV broadcasting to allow two transmitters on different frequencies to share the same antenna.
A "duplexor" is a device that allows both transmitting and receiving over the same transmission line or antenna. It may or may not be a passive device and it may or may not be frequency selective.
In WWII RADAR systems, a duplexor was used to allow the transmitter and receiver on the same frequency to share the same antenna. The duplexor was a waveguide device that had special gas-filled tubes to quickly switch the signal direction.
A typical ham radio cavity duplexor is used in FM repeater stations to allow the transmitter and receiver to share the same antenna. Since it is a passive frequency-based multiplexor, it is also a diplexor.
If you used the exact same device to instead connect two transmitters to the same antenna, it would no longer be a duplexor but it would still be a diplexor.
73, Tony AA2TX