The most common definition of is usually along the lines of the one I found on google [http://www.google.com/dictionary?aq=f&langpair=en|en&...].

> Terrorism is the use of violence, especially murder and bombing, in order to achieve political aims or to force a government to do something.

This was a single, possibly mentally unstable, man crashing a plane into a building. He may have thought he was trying to achieve something politically or to force the government to do something, but without any larger organization behind him it seems silly to label this as a domestic terrorist attack. It's giving the guy far more credit than he deserves.