This discussion got me thinking... yes, of course we know that in Dominion an attack card is anything that says "attack" at the bottom. But I agree with eHalcyon's definition that an attack is something that attempts to hurt others. But that's still subjective. I think we can quantify it, though...
If playing a certain card during a game causes, on average, an opponent's deck to be slower*, that card should be an attack.
*Slower is defined as taking more turns to accomplish the same thing (whether that thing is gaining 5 Provinces, or reaching some other sort of Engine that wins the game).
So with this, Masquerade and Tribute and Possession are not attacks because, on average, an opponent playing those cards will not slow your deck down. Of course, there are edge cases. There's always edge cases. Here at f.ds, we live and breath edge cases. Yes, Masquerade can slow you down if it forces you to pass a good card. Yes, Possession can slow you down if there's also Ambassador or Masquerade or Island in your deck. Tribute, I'd argue, is completely neutral, over 1000 games you should get just as many times that Tribute helps as the number of times that it hurts.
And of course there's edge cases in the other direction. Thief and Pirate Ship could quite possibly make your deck faster instead of slower. They are really dependent upon your strategy. But I'm willing to bet that if you took 100 different random simulator buy rules, checked how long it takes them to get 5 Provinces, and then played them against any official attack card, on average it will take them more turns to get 5 Provinces.