My former teacher, who had done her PhD in a top-10 university in the UK in 2009, is an assistant professor in my former school. On the day when I visited my former school, I saw her at the school canteen unexpectedly. I smiled and nodded to her before I said "Good afternoon, Professor Aiko." We were talking for a few minutes before she went back to her office. When I was sitting on a subway train on my way home, however, I was wondering if it was impolite for me to call her that. Can I call an assistant professor a professor, and say "Hello Prof. Nickname"?
Yes you can.
The "Assistant Professor" still teaches. The word Assistant is there to denote the rank within the academic system. Some have taught longer and are more accomplished and are rewarded accordingly.
Actually, calling the person "Assistant Professor Jones" would be very awkward and cumbersome. It should be avoided.
Yes, you can call any kind of professor a professor, and you should. Addressing someone as "assistant professor" or "associate professor" would be...well, it's simply not done, so I can't say if it would be rude or just weird.
It is similar to military protocol, actually: e.g. if someone is a rear or vice admiral, you call them admiral. In contrast to the above example, I'm sure some knowledgeable party could be specific about what bad thing could happen to you if you screw that up!
Yes, you can call an assistant or associate professor "Professor." That's completely normal practice. The only time it might be inappropriate is if you are writing them in a formal context. "Prof. Smith" is always OK, but saying "Mary Smith, Professor of Unusual Studies" isn't really appropriate if Smith is an assistant or associate professor.
This is not an answer but an anecdote about my current university. For whatever reason, someone decided that the faculty with Ph.D.'s should be addressed as Dr., whereas those with only a masters degree should be called Professor. There are some Ph.D.'s on the faculty who are offended if you call them Professor, despite the fact that they hold the rank of Professor. To me, this is much ado about nothing. I always caution my own students to be aware of our peculiar situation. However, I tell them to call me Mr., Professor, or Dr., whichever they feel most comfortable with.
(I had experience with two graduate schools in the US. In one the facuty were all addressed as Dr., in the other, all were addressed as Professor. Local norms and customs are hard to figure out!)
In Hungarian, we address only full professors as "professor". [Professzor úr! /Professzor asszony!]
"Mister/Ms teacher" would be the universal addressing for educators. [Tanár úr! / Tanárnő!]
(The exclamation mark goes with addressing in writing in Hungarian.)
"Mister Assistant Professor" would be awkward.
My impression is that, when someone is a professor and holds a doctorate, it's appropriate to use the "higher" of the two titles when addressing him or her. Which title that is depends on the institution. At some institutions, the people with doctorates are a proper subset of the professors; there "doctor" is the higher title. At other institutions, all the professors and some other people as well have doctorates; there "professor" is the higher title.
Of course, to follow this advice, you have to know who has a doctorate, and you have to know people's ranks, and you have to know what sort of institution is involved. In the absence of that knowledge, just use a title that seems appropriate. As Chris said, some people can be offended if you guess wrong, but I expect that those people are not very numerous. And, as far as I'm concerned, their being offended is their problem, not yours.
I am a visiting professor and this is not my primary role. I have asked the human resources department at the university how I should be addressed. They suggested that the verbal title professor is appropriate. However, when written, I should state that I am a visiting professor ie Professor Nickname, Primary role details, Visiting professor, University details. Actually I always invite people to use my first name anyway!
I would just go to the sports analogy. You have head coaches (professors) and assistant coaches (assistant professors). I can't imagine calling any assistant coach who has coached me "assistant coach". Just saying it is a bit demeaning and basically reminding them that they are just an assistant. I think at best if used you would seem a bit naive and at worst a jerk.
Yes, You can.
Hello Professor Nickname and Hello Assistant Professor Nickname: Simply compare these two and you will see that the first one seems more polite and normal. (However, culture of calling people and it's etiquette may vary in different countries and universities.)
However, I think that Hello Doctor Nickname or Hello Mrs./Mr. Nickname are good choices if you are in doubt about the politeness/correctness of using a specific title.
PS: This may be too pessimistic (and not so true) but I afraid, using Hello Assistant Professor Nickname may cause the person to feel that the student wants to remind that professor her level of profession (something like: you are an assistant professor not a professor) and this may cause her to be annoyed.
Another quirk: Here at a large Austrian university, my official German job title is "Universitätsassistent (postdoc)", which directly translates to "University Assistant (postdoc)". The official translation, however, is "Assistant Professor". And yet I don't have the rank of an Austrian Professor (neither "ordinary" nor "extraordinary"), and so German speakers would in fact be wrong to address me as "Professor so-and-so".
This is of course not only confusing to me. Since I also have a somewhat foreign name, students address me in all kinds of ways. I just got use to it as a fact of life.
Yes, being a Professor is a profession, the Assistant, Associate, or Full Professor just denotes rank.
At my school students also address lecturers as Professor in recognition of their role (and a sign of respect) which is perfectly fine and appropriate.
I'm in the US, and a professor (with rank of Associate).
The best thing is to just ask the person in question, some of my colleagues are rather informal, my cultural background is more formal. Consequently, different people will feel differently - you can't go wrong if you ask.
In Finland, in most fields, anyone would be immensely confused by any titles beside their names, usually their first name or a nickname, outside some extraordinarily official circumstances. Just to give further indication of how country-dependent this really is.
In our country, Prof holds a higher value when you are in the academe. We have many PhDs but only few becomes a full Professor. And nobody calls you a Prof until you become a Full Professor. You are not even considered a Prof if you are an associate or an assistant Prof. Come to think of it, You can earn your PhD by completing a program (4-5yrs) but to become a full Professor, you need decades and decades (minimum # of researh, min # of Publications in ISI journal, Min Number of Community Work, Min # of teaching experience, Min Number of presentationa, etc). Its very difficult to become a Full Professor that it holds more value than any position in the academe.
In Australia the convention seems to be the opposite of that described in most of these answers --- it is usual here to reserve the title of "Professor" only to someone who is a fully professor. It is certainly not offensive to call someone "Professor" if they are at a lower level, but it might feel unearned to the recipient.