Diplexer vs Duplexer. What is the difference?

A diplexer is a passive device that combines two inputs into a common output. The signals on inputs 1 and 2 occupy different frequency bands.

Consequently, the signals on inputs 1 and 2 can coexist on the output without interfering with each other. It is also known as a cross band combiner.

A duplexer is a passive device that allows bi-directional (duplex) communication of transmit and receive frequencies within the same band over a single path.