My definition is "Violence and intimidation, conducted by an organization not directly under the control of a national government, targeted deliberately at civilians, for a political purpose". Much of what is popularly referred to as terrorism is closer to guerrilla warfare.
I don't entirely agree with the linked post above, but I would also not call ISIS a terrorist organization. They may be major-league assholes who need to be dealt with somehow, but they're more of a statelet attempting to practice regular warfare, including controlling a specific territory, establishing their own political system, etc.
I think we would all be better off separating terrorism (a tactic) from identity (organization/state). Then we could categorize militant groups as whatever you want: state, statelet, militia, guerrilla force, freedom fighters, caliphate, NGO or whatever and separately discuss their tactics with respect to whether they commit acts of terrorism.
The message sent by terrorist actions is that your state is illegitimate because it cannot protect you from our tiny little group. It makes no sense for the state to send this message to its own citizens - it's goal is the opposite, by definition. If a state is directly trying to send the message to citizens of a foreign state that their state can't protect them from you, that's ordinary warfare.
I kinda like this as this separates attacks by these militants on military bases as "guerrilla warfare".
But what do you call when the attack is "targeted" at an armed force but with complete disregard to high number of civilian casualties? Examples: 1) Bombing a marketplace or mosque or any public place just because their might be SOME people or the opposing force there (apparently a lot of suicide bombings use this justification) 2) Missile strike on a UN school just because u suspect there are some members of the opposing forces hiding there (israel vs Hammas) 3) Drone strikes on villages to eradicate militants (with considerably high civilian casualties). (we all know who does this)
If the intent of the attack is direct attrition against an enemy force and supporting infrastructure, then it's guerilla warfare, even if most of the victims are civilians due to poor planning, bad information, etc. This may get particularly tricky in the case of suicide bombings in Israel - since virtually the entire country is part of the reserves, you could make the argument that any attack is an attack against the military infrastructure. I'd say that for it to be legitimate guerilla warfare, there must be at least a vaguely plausible plan for success. In the case of suicide bombings in Israel, they would then have to have a plan of killing the entire population of the country as a way of ending the fight, and their tactics would have to be a plausible way to carry that out. I don't think it fits that definition, so that would make it terrorism.
2 and 3 are ordinary warfare IMO, because they are both done by states and have the intent of attacking enemy forces. They may or may not be done with flimsy information, and the state in question and the militant group they are fighting against may both be in violation of assorted laws of war under various legal systems, but it's still conventional warfare.
I don't mean I agree with their cause but I am surprised attacks on service men & women don't happen more on their home soil which, from one perspective, is behind enemy lines.