Must an "academic reference letter" be from a university-employed researcher, or will other researchers do too? I'm applying for a post-doc where they ask for two "academic reference letters".  Does the adjective "academic" strictly mean they must be from people working in academia?  In my field, a good deal of research is performed at meteorological agencies, and I've co-authered several papers with a researcher employed at such an agency.  Would a letter from a researcher who publishes lots of papers (5 1st-author articles in 2 years), but is not associated with any university, be considered an "academic reference letter"?  Said researcher has a PhD, but further academic titles.
Better safe than sorry: check with the institution you're applying to, they may have formal requirements about this.

However, I don't think it's likely to be very narrow. The important points are that the recommendation letters come from people with a good track record, i.e. people who clearly perform high-quality research, and that they have worked closely with you in the past. Whether they're affiliated with a university, a meteorological agency, the NASA, a museum, or even (may God have mercy on your soul) The Industry… it doesn't matter much.