What is terrorism?

A simple dictionary definition:
terrorism:
noun
the use of violence and intimidation in the pursuit of political aims

That's a pretty vague definition. Its even more difficult to define in the real world when everything is rather gray, and not black and white, and its easy to see how that definition could mean different things to different people.
I was trying to observe the effects of terrorism on the mall and I could think was that the effects are only visible at certain times, and at those times they are highly visible. That's why I thought the incident with the Imams on the US Airways flight was interesting. Those passengers probably hardly ever thought of the war or terrorism, and then suddenly, that was all they could think of.
I suppose I've seen more soliders in the airport than I used to a few years ago, and more flags maybe, but not noticeably so like it was a few years ago. For awhile I saw more patriotic clothing, but now in DC the only patriotic clothing I see is those cheap souvenir shirts. In general though, I think that this is something people hardly think of, until something dramatic happens and then for a bit its all we can think of.