The nominal definition is a person or group that uses violence to achieve political goals.

The nominal definition is missing a key component: chiefly at non-military targets. So the suicide bomber that rammed into the the Marine barracks in 1983, Beirut was not a terrorist: he was a soldier engaging with the enemy. A suicide bomber, however, strapping on a bomb and setting it off in a busy marketplace is chiefly aiming at civilians and that is, in fact, definitively terrorism. And there were certainly a lot of actions taking place in the ACW that qualified completely and absolutely as terrorism (including the many concentration camps that held military and civilian prisoners indiscriminately and allowed them to die). (I don't know enough specifics about the American Revolution to know if George Washington was a terrorist or not. For some strange reason non-American countries don't go into squalid detail on that little tussle.)

But bombing or shelling a city indiscriminately like the US did many, many, many times starting from WWII and barely slowing down is definitely terrorism under any sane definition. So yes, the US military qualifies as terrorists under that generally-understood definition and is not openly called out on that because of, essentially, bigotry and fear of what the USA would do in return. And most US-backed coups and dictators were definitely terrorists. (Hell, a lot of Israel's leadership were openly terrorists aiming at civilians while carving their bloody piece of the "Holy Land" out.)

… it has never had a definition that has been consistently applied because that’s not what it’s for.

Here you spit truth. In the social sciences there are consistent definitions. In politics it's purely about the convenience of being able to label someone something bad.