As a formal title, "Professor" in most US institutions is the highest academic rank, above "Associate Professor" [frequently granted along with tenure after abo

As a formal title, "Professor" in most US institutions is the highest academic rank, above "Associate Professor" [frequently granted along with tenure after about 6 years and a successful assessment by the institution] and "Assistant Professor" [recently hired tenure-track faculty]. Also not uncommon is "Visiting Assistant Professor" [usually short-term non-tenure track faculty hired for 1-3 years as sabbatical replacements or to fill a (supposedly) short-term need]. Informally, all of these are referred to as, e.g., "Professor Smith" (much as a Lieutenant General might be called "General Smith" in conversation). Adjuncts (whose contracts are course-by-course rather than for any fixed length) are commonly given a title like "Instructor" or "Visiting Instructor", though there may be places that would assign a "Visiting Assistant Professor" title to an adjunct with a Ph.D. ("Lecturer" is less common in the US, in my experience, and often implies a longer-term non-tenure-track faculty position.) As often as not, students don't pay that much attention to whether a given instructor is an adjunct, so it's pretty common for them to be called "Professor Smith", too. So in the broadest usage in the US (which I think is pretty typical among people who don't work in academia), "Professor" winds up meaning "any college/university level instructor", while "Teacher" means "any elementary/middle/high school instructor".