TDMA is a digital mobile phone technology. In TDMA, the frequency band is split into a number of channels, which are stacked into short time units, so that several calls can share a single channel without interfering with one another.
Put another way, TDMA allows several devices to share the same frequency band at the same time, by letting each device "take turns" sending digital data. Each "turn" is called a "time slot".
TDMA, as a basic method of transmitting digital signals over radio waves, is the basis for several major wireless standards, including GSM, iDEN, and IS-136. IS-136 is commonly referred to as simply "TDMA".