In the United States, is associate professor a subset of professor, or a distinct category? In the USA, if someone writes that they are "professor" with no other specification, does this mean full professor, or can it also mean associate professor?  In other words, is associated professor a subset of professor, or is it a fully distinct category?
In the US, Assistant and Associate Professors are also titled Prof. X when being referred to formally. This is unlike in Europe and parts of Asia, where "Professor" is a title that can be used only if they're a full professor. Others are simply referred to as Dr. X (or Mr. or Ms. X, if they don't hold a doctorate).