Generically, the term TDMA can refer to any communication system that multiplexes a frequency using time slots (including GSM). In practice, however, this term usually refers to a particular implementation of TDMA, specifically, the IS-136 standard commonly used in the Americas.
With the announcement by the largest IS-136 carrier, AT&T, that it will begin a conversion to GSM standards, IS-136's future is in doubt. Other IS-136 carriers, including Rogers in Canada and Telcel in Mexico, have also begun GSM implementations. It is expected that these companies will continue to support IS-136 for some time as they phase in GSM.
The IS-136 standard is supported and promoted by the Universal Wireless Communications Consortium (UWCC).