Paul Cassell looks at a definition of 'terrorism' that I find problematic. What's central to terrorism, on this account, is causing harm to innocent civilians. That seems pretty far off to me.
First, it can't be just harm. It has to include threatened harm. Someone who threatens to blow up the Empire State Building if you don't fork over $1 billion is clearly engaging in terrorism, even if they never blow it up. Second, it can't be restricted just to harm to civilians. It's terrorism if you plant a bomb in the office of a high-ranking military officer. Third, terrorism doesn't always threaten harm, at least not directly. Eco-terrorism can include things like putting spikes on a road foresters need to use to cut down trees. No one might be directly harmed by this, and yet it's clearly a kind of terrorism.
What makes it terrorism is that the perpetrators intend to cause them to stop using the road by destroying their tires every time they do. Eco-terrorists have been known to cause far more property damage than that without causing direct harm, including blowing up people's houses because they work for a polluting company, choosing to do it when they're not home. That's even more clearly terrorism, and yet it's not about direct harm to civilians. I suppose if you have a sufficiently broad understanding of what counts as harm, then these might not be covered, but it's not intuitively the best way to get at what terrorism is. There are clear cases with no harm anyway, e.g. kidnapping in order to get some money or to get some political goal achieved when there's no intention of harming anyone, although there might be an implied threat of harm.
Haig Khatchadourian's The Morality of Terrorism opens with a very nice discussion of how to define 'terrorism'. He distinguishes between predatory, retaliatory, political, and moral/religious terrorism in terms of the motives, but all have one thing in common: there are immediate victims and a separate real target. There are lots of uses of coercion or force that aren't terrorism. What makes it terrorism is that the intended effect on the primary target is accomplished indirectly by doing something to an immediate victim who isn't the primary target. You do or threaten something to loved ones, civilians, a structure, and so on in order to get someone else to do something. That seems to me to be exactly what's definitive of terrorism.
The relevant section is also reprinted in the first two editions of James E. White's Contemporary Moral Problems: War and Terrorism. (I'm not sure why he removed it from the third edition, but what it does include doesn't strike me as being quite as insightful on this question.)