There's a bunch of whinging from rethuglikans that the Ft. Hood shooting is an act of terrorism because it was perpetrated by a Muslim.
Now I know it's asking a lot for rethuglikans to use proper definitions and all,but this is an important one.
Since I've already gone over the dictionary use of the word let's get into the legal definitions.
For judicial purposes you need a perpetrator or actor, a victim, and a motive.
There are specific conditions that must be met for all three things if something is to be considered terrorism.
1) The perpetrator(s)must not be acting on behalf of a government. People who commit crimes on behalf of a government are committing acts of either espionage or war, not terrorism. Perpetrators of terrorism are generally part of a non governmental organization of some sort, though that organization can be of just one person if other conditions are right.
2) Terrorism is an act or threat of violence against civilians. You can even stretch that as far as non-combatant government employees (see Oklahoma City) but a military base does not count as non combatant.
3) The motive MUST be designed to cause or instill fear for political gain. This is vital. Without this it really is just the act of a mad man. Scott Roeder, murderer of Dr. Tiller IS a terrorist because his goal was to terrify other doctors so that they would not perform abortions. That is terrorism.
John Allen Mohammed, the DC sniper who was recently executed, was not a terrorist. His ultimate goal was to kill his ex wife, though he made it look like a terrorist act so as to hide his tracks.
We don't yet know what the Ft. Hood shooter's motive was, but the Army doesn't think it was terrorism. Being that we know he fails the test on elements 1 and 2 (he was not a non-governmental actor and he was not firing on non-combatants) his actions, though horrible, were not an act of terrorism.
That's all people. Terrorism in a nutshell.