There is often a misconception among non-Japanese speakers as the the
weight that the title "sensei" is actually given in Japan. Basically,
anybody who teaches anything is called "sensei", even if you teach
bartending or (at an even lower level) English conversation. This
would include workers at day-care centers and your kid's soccer coach.
It's a term of respect (in that it's an honorific), but it doesn't
carry nearly the weight that "doctor" or "professor" would in the west
(although doctors and professors are also called "sensei"). Under
this system anyone who teaches an Aikido class, even if they
are a 5th kyu would be called sensei, and not many people would think
twice about it. Of course, not many people would attach much weight
to it either :-).
If someone's well respected you might call them "sensei" all the time,
even cross-discipline, or you might not, but there's nothing magic
about it -- it's just normal politeness, the same way that you might
call Tom Hank "Mr. Hanks" and not "Hey, Tommy" if you've just met
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Additional note from J. Akiyama:
Unlike the English language, the Japanese language places their
titles after the person's name. So, "Smith sensei" is the
correct way of using this title, not "Sensei smith."
Additionally, it's not very correct to call oneself "Sensei."
Whenever a teacher instroduces him- or herself to someone else in
Japan, they'll just use their last name. They might refer to their
profession as being a teacher ("What do you do?" "I'm a teacher"), but
they wouldn't call refer to themselves as such. So frankly,
introducing oneself as "I'm So-and-so sensei" sounds a weird and even
a bit presumptuous to my ears...