It's called, I forgot. Next time I will try to remember.
You have a whole internet at your fingertips with which to look up terms you don't understand without diluting the thread, but if you need at all to do that you should consider that you might not have anything relevant to add to the conversation to begin with. Hacker News is supposed to be a forum of educated professionals and domain experts, we shouldn't have to dumb things down here.
Otherwise said, using full expression is not "diluting the thread".
It's explicitly in the guidelines that we don't complain about voting because it "makes for boring reading." Complaining about acronyms has the same effect.
It is way better when you see a question and answer just below. And bonus point, question and answer teaches others to not use all acronyms they learned in all the random subcultures.
It doesn't, because we don't have to read that.
Also the interpretation can be gleaned from context, as @williamdclt did above. It's unlikely that, reading the thread, the "TFR" being discussed is "temporary flight restriction."