A method of two-way radio communication where devices on one end transmit using one radio frequency, while devices on the other end transmit using a different frequency. This allows simultaneous two-way communication without interference.
This is how most cellular phone networks are designed. Phones transmit to network towers using one frequency, while towers transmit to phones on a different frequency.
The alternative is TDD, or Time Division Duplex, where radios at each end "take turns" using the same frequency.