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.
I've never known of an academic program that didn't accept letters from employees at research laboratories. Depending on the field, demonstrating such contacts can be a significant plus. Of course, some academics are only impressed by other academics.