Okay, the title and the tags where done tongue-in-cheek, so don't go looking for your angrypants.
I've seen two big RPG releases in the last few years tout themselves as "open world", only for developers to come back quickly and clarify to the effect of...
It is open world, but it's not. The areas are big, etc.
The two recent games that I know of are DA: Inquisition and Witcher 3. Don't misunderstand, these are both great games and have the kind of large roomy areas that conveniently assuage any potential claustrophobia on the part of the player. However, these don't seem to fit the classic definition. i.e. You cannot run from one edge of the map to the other without opening a menu, which is conveniently called "map", in order to move from one "zone" to another.
Open worlds have: stream loading, most or all areas accessible from the start, may or may not have player-relative enemy strength. Personally, I find the games without mob level adjustment have the most charm. After all, nothing says hello quite like a "Welcome Bear".
Examples: Skyrim, Fallout, Most MMOs, older Might & Magic games and I remember at least one way back on the original PlayStation, but don't remember the name.
So, why are we seeing this term thrown around? Is it really just marketing departments seeing "valuable words" produced by the success of others and just too eager to apply them to things they're trying to sell?