I have a friend in the US military who said he was taught the definition "a military or para-military attack on a civilian target."
Seems pretty straightforward; though when I responded by saying that the US military definition would qualify the bombing of Hiroshima and Nagasaki as one of the worst acts of terrorism in world history, he got pretty pissed off.
Is the civilian population that supports the war through manufacturing, agriculture, finance, communications, logistics or intellegence part of the infrastructure?
If they are, then the World Trade Center would have been considered a valid military target.