Yes, in most definitions or terrorism, the key part is (trying) to create terror to further your goals. It doesn't really matter whom you'd attack, be they civilians, structures, organisations, armed forces, or whatever. The difference between terrorists and freedom fighters and (national) armies becomes blurred fast, though.
If people would just quit being afraid of this stuff, by definition, 'terror' wouldn't be created, would it?
Telling people the rapture will happen at 2:30pm tomorrow would in fact create terror in the minds/hearts of certain people. Are you a terrorist if you tell people that Jesus is coming back tomorrow afternoon?
Yes, terror is certainly in the eye of the beholder, although academic definitions are a bit more nuanced than those being used by the public.
The subtle difference is between using terror to reach a goal and using some action to reach a goal with a side effect that people experience terror. For example:
A group of hackers could hack into the bank accounts of the 1% to distribute their wealth among the other 99%. They don't have any intention to create terror and probably think that the 1% can easily take. Of course, the 1% will see it as an act of terror. And probably journalists, lobbyists, politicians will spin it and use it to create terror among the larger populace.
Another group of hackers is hacking into facebook accounts to make people's secrets public to try to get the public to care about privacy and not to put their trust blindly into social media. In this case, they would use terror consciously as a means to this end.
Yes and no. Getting shot is terrifying to you, but the other army isn't shooting to terrify you, but to incapacitate you and your colleagues to reach some (strategical) goal. The terror is just a happy side effect. On the other hand, armies could also use acts of terror to reach some goal. For example, instead of just taking soldiers prisoner of war, you could just cut off their heads and put them on a stake at the front. Or raping all the women in the occupied territory (although that could also be a way to 'lay claim to the land and people' by creating a generation of mixed bloods, I suppose. In that case, it wouldn't be an act of terror, although everyone on the receiving end would be plenty of terrified)