Time Division Duplex
A general method for two-way communication where devices at each end of a connection "take turns" transmitting on the same channel.
In radio networks like cellular phone networks, this means taking turns transmitting on the same radio frequency. Typically, the "turns" are so rapid - hundreds or thousands per second - that a human could not possibly notice them during a voice phone call, for example.
TDD was once rare for phone networks, but has become more common, particularly with newer, higher frequency bands.
The alternative to TDD is FDD, where two separate frequencies are used for communication in each direction.
FDD requires paired radio spectrum.
See: Paired Band