What is technically considered an attack?

[Rebonack]

Well considering how much this edition has gone the way of being REALLY technical in its rulings we can make this work.

The ability actually says "until it attacks" not "when it makes an attack" or similar language. In this case one could argue that the word is not the game term of attack but is the more broad term of general English of "attacks" and thus could be ruled to include things that are not just what the game terms is strictly an attack.

Really technical sure but if you want another example of really technical language making a ruing look no further than the magic initiate ruling on whether you can use the spell with your spell slots (ruled to only work if it is on your class's list) or why the various different +stat to spell damage abilities work differently depending on their exact wording.

The magic initiate ruling is still pretty mind-warping to me. I can see it, though. Sure you're learning it, but in all class features that have similar effects they also specify 'And it becomes a (Class) spell for you' while the Magic Initiate feat lacks that clause. For the +Stat effects, I think most of them have been irrata'd to include (or not include) something that it specifies whether or not it only applies once per turn. The dragon sorcerer, for example, applies bonus damage once per turn. The phoenix sorcerer lacks that clause. They caught it on the Celestial Warlock vs the Undying Light Warlock, too.

And for running with technicality, I can see that too. The Imp's invisibility action states 'Until it attacks' as opposed to 'until it uses an Attack'. Though the invisibility SPELL uses the same language, so it feels like a stretch to me. Also probably worth mentioning that this was something of a compromise. The game at the table is Princes of the Apocalypse and the party found a Wand of Magic Missile pretty early on with some pre-set loot. The Warlock player spotted the interaction (or rather lack of interaction) between the Wand and the Imp's version of invisibility and made the pitch to me. I vetoed it, but to keep 'attacks' mechanically consistent I conceded that Hex would be able to trigger off basically any damage source.

In the end, the Warlock netting an extra 1d6 Hex damage off Fireball has proven to be the MUCH lesser of two evils.