In late 2001 Richard Sharp mentioned the idea that “a terrorist ceases to be a terrorist when his cause is just”. I can't swallow this; the justice of any cause is highly subjective, and should be irrelevant to a definition such as this.
I propose that a terrorist is anyone who deliberately kills or injures innocent civilians because they are members of a country or race or other large group against which the terrorist has some grudge.
I exclude from the definition of a terrorist anyone who targets soldiers or politicians and kills innocent civilians only by accident. This exclusion is for three reasons:
By my definition, the people who destroyed the World Trade Center were terrorists. Various Palestinian and Northern Irish gangs are terrorists. Israeli government policy has probably been terrorist from time to time, but the recent policy of targeted assassination isn't terrorism (whether it's right or wrong is another matter), because it's aimed at people who are asserted to be individually guilty.
In the Second World War, both sides openly practised terrorism by bombing each other's cities indiscriminately. The French Resistance wasn't terrorist as long as it stuck to killing German soldiers, though I think it sometimes found other targets.
In Spain, ETA mainly targets people with whom it has some quarrel, but these include journalists as well as soldiers, politicians, and judges; and recently random civilians have also been hit. So I think it's fair to call ETA a terrorist organization.
My definition of terrorism, like any normal dictionary definition, is morally neutral: it doesn't say whether terrorism is right or wrong, it just defines what it consists of.
Personally, I feel that terrorism is always wrong. But, of course, to say that someone isn't a terrorist doesn't mean that he's right. If the IRA or ETA limited themselves to killing soldiers, they wouldn't be terrorist organizations by my definition, but in my opinion they'd still be wrong.
The desire for self-determination is in general moral and right, but it doesn't in itself justify killing anyone. The only people who deserve killing are those who have killed others.