what is the difference between duplexer & diplexer?

1) A duplexer is generally a 3 port device (for example a circulator) that permits you to have RX and TX at the same time by using the same antenna

2) A diplexer is generally a 3 port device that permits to multiplex 2 bands incoming at the input port over two different outputs, each band is forwarded to one output (for example a diplexer could be used to sepatate tv+satellite bands over the same coaxial cable into two different cables).