Can you please clarify backbiting? At what point does it become this?

So the general principle of backbiting is if you were to say something about somebody else who's not present in the conversation and they were to have heard that, then they would not like it. That's backbiting. So for example, Muhammad says to Abdullah, did you know Zaid did X, Y and Z? He's like X, Y and Z, for example. If Zaid was to have been there and to have heard that and he would not like it, that's considered backbiting. Now, if Abdullah, for example, in this example, does not know who Zaid is, so Muhammad says a person did it, that's not backbiting. But if he knows specifically who Zaid is, then it's considered backbiting. There are certain exceptions. So if you're doing it for, let's say, in therapy where you need to speak about the other individual or there's a specific legal case going on or fiqh ruling, for example, or you're speaking to a sheikh about something, those are more exceptions. But generally speaking, the blanket rule is that you should not say something about other people apart from that which they would like.