An old scientific principle says, "instead of asking what it is, just ask 'what does it do?'" It's a lot clearer if we speak of methods.
Terror tactics are defined as those that are aimed at undermining morale and causing (as the old WWII officialese had it) "alarm and despondency," rather than causing strategic damage. Blowing up the Pentagon, if Al-Quaeda had achieved it, would have been a plausible military tactic: blowing up the WTC was a terror tactic.
Terrorists, as commonly defined, are guerillas that use terror tactics. The Talliban in Afghanistan, for the most part, seem to focus on military targets; a roadside bomb against an army vehicle is not a "terrorist" weapon, just a shot in a guerilla war. (Almost identical devices were used by the Resistance in WWII Europe.)
When national governments use terror tactics, it's usually not admitted as such. When the British Army responded to Ghandi's protests by shooting down a square full of peaceful demonstrators (for what the commanding officer later admitted was the "moral effect," i.e. the terror value,) they argued for months that it had been a riot action. When Israel indiscriminately shelled Gaza, focussing on hospitals, power plants and the like, they still maintain the fiction that they were targetting "terrorists." "Operation Shock and Awe" in Iraq was remarkable for being explicitly named as a terror tactic, but since (like Israel in Gaza) they also used banned weapons such as white phosphorus, they clearly felt they had a free pass to break every rule.