Nowadays there are many books, teachers and centers teaching
vipassanaa. Of course, they use the word to designate a certain way
of practicing. But how is the word vipassanaa used in the original
sutta-s?

It's actually a relatively rare word in the sutta-s. The most often-
mentioned meditation practices there are sammaa-sati and sammaa-
samaadhi.

And when the word vipassanaa does appear, it's generally in the pair
samatho ca vipassanaa ca (e.g. DN 33).

The DN commentary glosses these as samatho = samaadhi, and vipassanaa
= pañña.

But in SN 43.12, samatha, vipassanaa, samaadhi and sati are all
treated separately -- as though they were distinct.

In SN 35.245 samatha and vipassanaa are compared to a "swift pair
of messengers."

So, in the sutta-s themselves, what exactly does vipassanaa mean? And
why the samatha-vipassanaa pairing as well as samaadhi-sati?

Derek.