Being a fan of old-school fully rigid 26" machines for their simplicity and lighter weight, I've often pondered the correct term myself. I have, amongst other things, a couple of old Raleighs, a Mustang from around '89, and a Highlander from '91 and they both have "all terrain" and "ATB" on their frame decals. I probably use the phrase MTB more often, but regard ATB as the more correct term if talking about machines from the era where rigids were the norm and many of the frames were still lugged & brazed. MTB implies use on mountains, whereas ATB implies use literally anywhere you can ride a bike - and the current breed of discipline-specific full-sus MTB's are far less suitable for general purpose use than an 80's/90's ATB whose DNA was really a beefed-up roadster/tourer geometry frame fitted with wider rims and knobbly tyres.