I believe the best term (in the abstract) is "adjunct professor." "Visiting" does not seem to apply, for the reasons you state.
"Adjunct" conveys some useful information, and is important in the politics of how academic departments function. Many adjuncts do not have professional ambitions in academia; they are well established in their professional field, and teach a course in order to stay in touch with the academic world, to add credibility to their name, or similar reasons. So, while from an academic perspective an adjunct might be considered less academically qualified, from their professional perspective that might not be especially important. Also, bringing practical experience into the classroom is highly valued in many disciplines.
There has been a great deal written in recent years about the efficacy and ethics of universities, community colleges, etc. hiring more adjuncts, at the expense of tenure-track professor positions. They do so, in part, to save money, which is seen by many as an infringement into the academic quality/integrity of a university. Worth looking into.