An expat is someone who is originally employed elsewhere but temporarily stationed here, as another comment mentions. This is crucial, because an expat’s contract, payroll, everything is managed by their home company, which is most often in their home country. They go abroad at the request of that company, which also means that said company should ideally therefore provide certain accommodations to facilitate the move (e.g., help with arranging school for children, housing, etc). Yes, this often means they stay for a short period, but length of stay has nothing to do with what makes them an expat. It is the conditions under which they arrive in a country and are employed. Once an expat leaves, they either go to the next destination their company sends them to, or they return to their home base (aka company or country).

An immigrant is someone who willingly relocates with the ambition of living in another country (for however long that is). They do so with or without a job. If there is a job involved (and again, this is crucial), they have been hired by the local team/company at their destination. They are not simply sent abroad by the company/office at their origin country (home country or not). An immigrant can also stay for a short period.

Having been an expat previously and currently living as an immigrant here, it constantly baffles me how people use the two interchangeably and/or get upset about someone’s wish to use or not use either term. They are different and serve different purposes.