The nominal definition is a person or group that uses violence to achieve political goals. By that definition, the US military, George Washington, and all US backed coup leaders are terrorists. Of course, the US would never call them that because it would expose the contradiction. It is and has always been selectively applied to America's enemies.
There's another aspect to this. The AUMF gave the president complete military control with no restrictions to fight the "War on Terror". So when Maduro and the random boats in the Caribbean were labeled "narco-terrorists" it was to give a flimsy justification for the president to use the military however he wanted.
"Terrorist" can mean freedom fighter. It can mean revolutionary. It can mean political rival. It can mean peaceful people with the wrong political beliefs. It can just mean fisherman. But it has never had a definition that has been consistently applied because that's not what it's for.