There's recently been some debate about the meaning of the term "open turn". As "open" is a word which has been implied to so many different meanings in ballroom we may not be able to come to complete agreement, but I thought perhaps we could try.
Presumably, those turning figures that call themselves open are open turns. So for example, a Quick Open Reverse, and an Open Natural should both qualify, along with their American Smooth identical cousins the Open Left Turn and Open Right Turn.
One poster seems to suggest that the modifier "open" is used to imply that the feet do not close, but rather pass. I had accepted this usage earlier today, but now I think I need to disagree (if that is actually what the poster was arguing in the first place).
There are another class of turns which feature passing footwork, but are very clearly not open turns - these would be the natural and reverse followers heel turns of basic international foxtrot. In contrast to the open natural and open reverse, these are inline figures.
With the contrast to inline as the clue, I think it become clear that the meaning of "open turn" is that it involves or prepares an outside partner action. In the open reverse turn this happens on step three. The open natural prepares for an outside action to follow on step four. If you think of the figure as repeateable by interchaning roles, this is the same as saying that the open natural features outside partner on step one - only we might overlook that as the figure is most often commenced from promenade.