Ask 100 brewers this question, and you'll get 100 different answers. Here's mine:
If it tastes like a lager, you can get away with calling it a lager.
That being said, I've tasted many dozens of so-called "lagers" and "lager-like ales", and to my palate........ only once or twice did they truly taste clean enough to be real lagers, IMO. Sure you can get close, but close enough for one is not close enough for everyone, in my experience. Maybe I'm just real picky, but it has to be squeaky clean and malty to be a "real lager". My opinion.
Also, if it's fermented with a lager yeast at cool temperatures, you can definitely call it a lager. Even if it has flaws, at that point, well you've still made a lager! Just maybe not a very good one!
Anything else, ANYTHING else, in my opinion, is really an ale, or a hybrid style where lager yeast was fermented warm like a "steam beer" or whatever.
Yeah I know my opinion stinks. But you'll only get opinions out of us. There is not any "right" answer on this, except in each of our own eyes.
Or perhaps the one right answer is: pastorianus yeast fermented at about 52 F or less for several weeks. That's definitely a lager. Anything else is left to the tastebuds of the beholder.
Cheers.