I had never hear the term"expat" either before living in Cambodia. Like you, I guess I had maybe encountered it in a textbook before and had a rough understanding of it's meaning. After 10 years in Cambodia I now associate the word with entitled western retirees and trustafarians who don't need to participate in the local economy for their own sustenance. I definitely prefer to associate with "immigrant" and "migrant" because these terms seem to identify what your goals are in your target country. "Expat" seems to mean the most important thing is that you are abroad. There is a lot of colonial mentality within many of the people who identify with this term.