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 is rare for phone networks. The much more common alternative is FDD, where two separate frequencies are used instead of one.
Although FDD is the preferred configuration for a cellular phone network, it requires paired radio spectrum.
See: Paired Band
Therefore, TDD is used when a carrier needs to deploy a new network or additional capacity, and the only spectrum available is an unpaired band.