Can you please explain in what context the terms are used. For instance FDMA means Frequency Division Multiple Access and refers to the typical system of multiple carriers operating independently on different frequencies. When I hear the word multiplexing I usually think of independent information encoded on the same carrier.
However with the term TDMA, Time Division Multiple Access, the difference is not so clear. In fact with TDMA the two terms are really synonymous because all the time slots are encoded on the same carrier.