There are some articles of the Geneva convention that explicitly bar indiscriminate attacks on the civilian population. One definition of terrorism is a violation of those articles. This is probably roughly the definition Brad has in mind.

A tighter definition is etymological: it's not terrorism unless you're trying to terrify people. By this definition, a man who blows up a city because he's trying to frighten other cities is a terrorist, but a man who blows up a city because he wants the people in the city to die, or one who just likes watching mushroom clouds, is not a terrorist.

We often see statements like "terrorists blow up Cole" which are technically true though a little misleading. The people who blew up the Cole probably were terrorists, in the sense that they carried out terrorist actions at other times, even if the attack on the Cole wasn't terrorism.

These definitions probably aren't capturing what we really care about. Perhaps the key issue here is that we don't care whether they are terrorists. We care that they are killing innocent people and want to establish repressive obscurantist governments, and this is where our terminology should focus.