It’s a nervous tic of analytical philosophy to be forever wishing to clarify distinctions that nobody is actually confused about.
Terror is a tactic used successfully by states and groups alike. (See the excellent paper “Terror’s Mask” by Michael Vlahos of the Joint Warfare Analysis Dept, Applied Physics Laboratory, Johns Hopkins University.)
Its essence is violence to civilians; any weapon qualifies.
The threat of terrorist attacks is as much terrorism as the attacks themselves. (Terrorism works by instilling terror.) Nuclear weapons are useful for little else; states that own them are ipso facto terrorist. That includes the United States, the United Kingdom, Russia, China, Israel, Pakistan, and India.