Where was the term "Action item" was first coined, and isn't just a fancy and euphemistic way to say "task"?
It's simply a way of saying that someone is expected to take some sort of action to resolve an issue.
This Ngram seems to show that the term started appearing in print in the early 1960s. I did a Google books search, and one of the earlier usages I found was in a 1967 NASA report. (This was hardly surprising, as I worked for the U.S. Government for quite some time, and the term was well-used in that context. I've sat around many conference room tables discussing issues that were turned into action items, and then around that same table a few weeks later hearing people give a status report on their action items. I have no idea if the term started in the government and then bled into industry, or vice-versa.)
I also found the term formally defined in a 1965 Government report.
Also, I'm not sure how widely the term is used outside the business world, but, nowadays, it seems to be rather commonplace in arenas such as project management, as evidenced by this book.
I suppose you could say "task" (or "tasking") if you wanted, but I think task is more generic and perhaps could be applied to a wider range of activities or jobs, like, "Make 10 copies of this report." I think action item implies at least a little bit of legwork or research will have to be accomplished.