Excellent question! If such an attack happens this year, I'd say it wasn't a terrorist attack, but if mattnewport felt that it was I'd pay out without making a poll.
I'd lean towards saying it was a terrorist attack but I'm sufficiently uncertain about how to classify it that I'd be happy to let a community poll settle the question.
I would like to propose this as a thread for people to write in their predictions for the next year and the next decade, when practical with probabilities attached. I'll probably make some in the comments.