Realistically, Mountain Bikes was a company that made these new fangled machines- but the phrase stuck and kind of like "Kleenex" or "Xerox" and to some extent "Coke" the term became what the thing was called. I like to consider those old bikes are sort of more technically called ATBs "All Terrain Bikes."

Takagi Tourney AD and Tourney XT cranks were some of the more common higher end cranksets found on early mountain bikes. The M700 Shimano Deore XT "Deerhead" components you have are from Shimano's first high end mountain bike groupset.