The word "terrorist" (or "terrorism") is like any other word of the English language, which means that the notion of an "authoritative" definition is a bit sketchy. One finds definitions in dictionaries, and I suppose an authoritative definition is found in an authoritative dictionary. Your desideratum of an authoritative source precludes using Urbandictionary. Since there is no one official dictionary of English, you would have to go for one of the most respected dictionaries: The Oxford English Dictionary, or Merriam Webster's Third New International dictionary. It would in fact be best to provide both, since this could reveal differences in sources (and what they have in common).
Another approach would be to gather together statutory definitions. This is generally in inferior approach, because it does not correctly say how the word "terrorism" is defined, but if you were explaining to an alien lawyer how we statutorily define "terrorism", then of course you want to point to specific statutes. There are enough Anglophone countries that a comparative study could be interesting. It really depends on why you want to "define" terrorism/t.
At the US federal level, "international terrorism" is given a definition, applicable to Title 18. A seeming peculiarity is that the act must
occur primarily outside the territorial jurisdiction of the United
States, or transcend national boundaries in terms of the means by
which they are accomplished, the persons they appear intended to
intimidate or coerce, or the locale in which their perpetrators
operate or seek asylum
The reason for this is that, unlike many countries, there are federal and state levels of law-making, and under the US Constitution they are supposed to have distinct subject matters (hence regular robbery is not a federal crime). The aliens would have to understand this feature of US law, in order to understand the otherwise "incorrect" clause in the Title 18 definition. There is a different definition of terrorism under Title 22, pertaining to the obligation of the Sec'y of State to report on terrorism, which is
premeditated, politically motivated violence perpetrated against
noncombatant targets by subnational groups or clandestine agents
and "international terrorism" is "terrorism involving citizens or the territory of more than 1 country". For purposes of the Attorney General, the Code of Federal Regulations semi-defines terrorism by saying that
Terrorism includes the unlawful use of force and violence against
persons or property to intimidate or coerce a government, the civilian
population, or any segment thereof, in furtherance of political or
(Other things could be considered "terrorism", so this is an "at least" definition).