In our hobby, it's whenever the airplane stalls and "drops a wing". And they almost always drop a wing, no matter where the stall starts on the wing, tip or root.

Straight wings naturally stall at the root. It's how they do it. But stalls start at the tip on tapered wings. Almost never do our wings stall equally on both sides at the same instant. So what happens? One wing loses lift before the other and the airplane drops a wing.

And every time one of our models drops a wing everyone talks about it as a tip stall. An Ugly Stik did it the other day. Probability is close to 100% that straight wing stalled inboard first, but one wing went before the other. Everyone called it a tip stall. The term is great for describing the results, but sucks at describing more than that.

BTW, where the stall starts has a lot to do with how quickly the results show up. It's simple leverage or lack of it. When the lift toward the tip is lost, it has a more pronounced effect than loss of the same amount of lift would have closer to the fuselage. So you often see tapered wing airplanes drop a wing much faster or more violently than you see the wing drop on a straight wing airplane. It's also easier to stall the shorter chord wing tip. Shorter chords are less effective than longer ones and stall sooner. Which is why tapered wings stall from the tip. And why straight wings don't.