Like Jackie I thought it might be a leftover from the days when refrigerators/freezers had a defrost cycle. Then I had my doubts and searched for +"frozen food" +tips +(defrost OR thaw) and scanning through the first 30 or so applicablle hits found the two words seldom appear in the same hit and there doesn't seem to be any pattern e.g. some say thaw in the microwave and others say defrost in the microwave. So, I returned to my first guess that "defrost" is an artifact of language that came with older appliances.