As I understand the US system for four year universities
TOP Professor with an endowed chair University or Full Professor
Associate Professor (They have tenure and time in position)
Assistant Professor (They don’t have tenure. This is their first job and they want tenure here.)
BOTTOM Visiting Assistant Professor (They got denied tenure at a higher ranked university than they currently teach at and want tenure at that university or another one of similar rank.)
Adjunct Professor Someone with a non-academic job, expertise or position who teaches a university course because they enjoy doing so. Also a deluded holder of a Ph.D. who thinks this is still a part of academia. It is not. That is why it is below the bottom. These people are highly qualified, poorly compensated casual labour, desperately clinging to hope of an academic career after this makes any sense.