The US Geological Survey Manual says this:
Mountain Flying: Helicopters.
Conducting flight operations in mountainous terrain including pinnacle landings and approaches at varying elevations and pressure altitudes of over 5,000 feet above sea level at temperature ranges above 75ºF, and in areas of rugged peaks, deep canyons, cliffs, rock outcropping, steep slopes; including landing on mountain tops and confined areas surrounded by trees, brush, rocks, snow, or ice.
Found it here:
http://www.usgs.gov/usgs-manual/hand...-2-h/a275.html
But then again, they also talk about pressure altitudes above sea level - and it looks like if the temp is below 75f its not mountain flying either. I guess Everest is just a hill to the USGS. I did find something once on either the USFS or OAS websites that defined their idea of mountain flying - something quite sensible that referred to a certain increase in altitude over a certain distance. I think i'll try to look it up again.