Open world means that it's a seamless world, it has nothing to do with progression locking. GTA has always been open world yet it was gated by story...
IMO, Open world means it is a world that is entirely open, always. Seamless means it is seamless. Open does not mean seamless in any definition of any dictionary I know of, tbh.
Anyhow, genre definitions shift and change with the times, I think. I really believe ESIV: Oblivion was the catalyst for the shift in the definition of the Open World terminology, blowing that genre wide-open and setting the standard. GTA
was gated by story, but in the wake of Oblivion immediately dropped it (4, Chinatown Wars, Episodes and 5 all have the complete map at your disposal from minute one), The Witcher is the same, even MGS5 is the same; MGS5 unlocks a second, different area around halfway through, but each of those seamless areas is completely open to you from the moment you step foot into it. Now, some areas of the maps in Skyrim or MGS5 might be unwise to visit in your first sitting due to your character level or the equipment at your disposal, but the game doesn't stop you from trying - I get the impression FF15 will be more heavily gated, and that's fine.
Is Ocarina of Time open world? By modern standards, I would say it isn't, but it is one large seamless world, with progression gated by story progress and item possession - so by the definition you provided, it is... but I wouldn't call this truly open world; it's open-ended. But open world - these days - is much more, if not entirely, free of these sorts of barriers.
Also: Games can be open world without being seamless, I think. Chances are, for instance, Dragon Age Inquisition is more 'open world' than FF15 will be in that each discrete area is very aimless and 'go forth' in design, but the flip side of that is you're still selecting different areas from a menu that's a world map. (Incidentally, this approach brings up something I've been thinking about regarding FF15 for a while: with one seamless landmass as the world map, presumably the climate and temperament won't change that much. IE, there's little chance of something as disparate as an 'Icicle Inn (FF7)' moment or a Desert Palace (FF9) or something; being on one continent does that. Which is a little of a shame to me, but it's okay. FF10 has this too, being a more enclosed story rather than a globetrotting one.)
Basically... I think there is a definition and a distinction between open world games and games with open-ended design; I would describe Witcher 3, Elder Scrolls, MGS and modern GTA as truly open world, but I would describe stuff like Xenoblade, Dragon Age, and it's looking like FF15 as having open-ended design but not truly open world in that sense.
I think this is why they've backed off from the terminology open world in interviews and stuff. They said it a lot early on, but a few months later Tabata was saying 'It's open world, but if it were totally open world, it'd defeat the point of being a FF' (paraphrased) and "it won't be open like Skyrim" etc.
I don't think this is bad in the least. They're focusing on where they're comfortable and what they know. I feel like the open world buzz word was deployed a lot form a PR perspective early on (and even during LR's promotion) because Japan at the time (and still to an extent) was chasing Skyrim after seeing that as the 'future' of RPGs (something Kitase outright said) - but I think they've now realized they 1) can't make a game like that currently and 2) have strengths of their own outside that, and the open world chatter is really just a hangover from earlier in the PR cycle when the messaging was different.