For most English speakers, sensei entered everyday vocabulary through the martial arts rather than through Buddhism: judo, karate and other Japanese martial arts spread widely in the United States and Europe following American occupation and military contact with Japan after the Second World War, and the instructors of these arts, whether teaching in Japan or later training students abroad, were addressed as sensei from the outset, well before Zen meditation centres using the same word for a Buddhist teacher became widely established in the West.
As a result, most Western encounters with the word sensei today carry no religious content at all: a karate or judo sensei is simply the instructor of a physical discipline, addressed with the same term of respect a Buddhist student uses for a meditation teacher, but conferring no religious authority and requiring no religious training whatsoever, a complete separation of the honorific from any Buddhist content that mirrors, in miniature, how broadly the underlying Japanese word functions in ordinary usage, applied to schoolteachers and physicians as comfortably as to any kind of spiritual teacher.
The word extends just as naturally to Japan's other traditional arts, so that an instructor of tea ceremony, ikebana flower arrangement, calligraphy or traditional dance is addressed as sensei with exactly the same respect and exactly the same absence of any specific religious content as a martial arts or academic instructor, underlining that the honorific tracks recognised mastery and the role of teaching itself rather than any particular category of skill, spiritual or otherwise, that the tradition being taught happens to involve.