I think the definition of an attack should be "any kind of card that has an usually undesirable effect on your opponents, where 'usually' means 'ignoring interaction between kingdom cards'". Council Room, Masquerade and Possession only give a negative effect provided certain other kingdom cards (like Pillage, Militia, Baker), and all attacks are only non-negative on boards with certain other cards (reactions, draw-up-to, etc).
Keep in mind that only 3 of oh so many Dominion cards can actually block your card. Ignoring them, your card will always harm your opponents, and this is enough for an attack type.
Council Room does give a negative effect in a single-card kingdom, too, it triggers unwanted reshuffles. Masquerade sometimes makes you pass a good card even without hand size reduction attacks and Possession can make you skip a good hand. And Margrave, Minion, Soothsayer, Thief, Fortune Teller, Saboteur, Rabble, Ghost Ship and Pirate Ship can help opponents without the presence of any other cards, even if you're not trying to help your opponent with them. "Undesirable" is relative, so basically what your definition actually is, is "it should be an Attack if the card feels like what the word 'attack' means" which is a flavor reason.
There is no reason to have an Attack type if there are no cards that interact with the type Attack. If there was, there also should be a Splitter card type, a Trasher card type, a Virtual-Coin type etc.
Hmm... Good points.
Disagree, those weren't good points.
eHalcyon has already basically made the case, but I think these arguments in favor of not having the type "attack" require some serious pedantry. I find it completely irrelevant to point out that it's theoretically possible for a play of Council Room to hurt someone. This isn't a question of opinion or subjectivity. "Undesirable" is not subjective. Actual numbers can be used here.
How about Awaclus and I play a bunch of rounds of Dominion, with Council Room in the Kingdom. Except whenever I play Council Room, he doesn't get to draw a card. But when he plays Council Room, standard rules apply. There is absolutely no realistic chance that if he will win many games in which we both buy Council Room. Same goes for Witch... since Curses can theoretically help you, how about whenever you play Witch, I don't get a Curse, even if I want one. But when I play Witch, normal rules apply. We can run a modified sim if it's really needed, but I think it's quite obvious what the outcome would be.
An attack in Dominion is a card that has a harmful effect for your opponent. A harmful effect is one that, on average, will cause your opponent to need more turns to score a given number of points than they would have otherwise. That's it. The only exception to this is IGG, which as explained, would cause a lot of rules confusion if it were labeled "attack."