Who , when and where was the term 'exoplanet' coined?
I very much doubt it would be possible to know this sort of thing with any precision. The best I can do is note that in searching on the Astrophysics Data System, the earliest published occurrences of the term are from 1992, e.g., a short article entitled "Searching for exoplanets" by Bernard Burke in what was probably a conference proceeding. (The actual article is not available, alas.) Some of these papers use "exo-planet" instead of "exoplanet", which suggests the term was new enough for people to still be uncertain about how to spell it.
This 1992 report from a meeting of the Royal Astronomical Society mentions a talk by Burke in which he "... described the work of the NASA Science Working Group on Exoplanetary Systems between the years 1985 and 1992..." So it's possible the term (or at least the adjectival form) goes back to the 1980s.
After an extensive Google search, I found this. Quoting last sentence of 4th paragraph:
It is only in the last 25 years that we have been able to say with complete certainty that these planets exist, so the term “exoplanet” was coined to describe them.
Obviously, the last part of the sentence refers to the future of the moment before 25 years. It was written in 2018. Thus, the term exoplanet was coined around 1993.
You can believe me, I have done lots of googling, and I can tell you that the name of the person and the place are almost certainly unknown. But there is still a possibility that the name is actually known. I personally believe that IAU has come up with this term as an organization, but I am not sure because it is not stated anywhere explicitely.
Another name for exoplanet is extrasolar planet. After some thinking I figured out one possibility: When the people at IAU used this term often, it became hard to say it fast. Somebody came up with the shortened version exoplanet and that quickly stuck as term for this. They started using it gradually, so the name of the original person is maybe lost. So, maybe that was once just jargon at IAU, but that quickly got stuck in media and also in general audience.
I can just add a little additional information: Google ngram shows a nice exponential grow for the term "exoplanet" starting in the mid-1990s.
Fun fact: The earliest exoplanet HD 114762 b was announced in 1989 according to NASA's exoplanet catalog. The original article is paywalled, so I am not sure whether the authors used the term "exoplanet" back then.