By any definition, that is an act of terrorism.
No, it is not.
Proposed Comprehensive Convention on International Terrorism
Since 2000, the United Nations General Assembly has been negotiating a Comprehensive Convention on International Terrorism. The definition of the crime of terrorism, which has been on the negotiating table since 2002 reads as follows:
"1. Any person commits an offence within the meaning of this Convention if that person, by any means, unlawfully and intentionally, causes:
(a) Death or serious bodily injury to any person; or
(b) Serious damage to public or private property, including a place of public use, a State or government facility, a public transportation system, an infrastructure facility or the environment; or
(c) Damage to property, places, facilities, or systems referred to in paragraph 1 (b) of this article, resulting or likely to result in major economic loss, when the purpose of the conduct, by its nature or context, is to intimidate a population, or to compel a Government or an international organization to do or abstain from doing any act."[36]
That definition is not controversial in itself; the deadlock in the negotiations arises instead from the opposing views on whether such a definition would be applicable to the armed forces of a state and to Self-determination movements. Thalif Deen described the situation as follows: "The key sticking points in the draft treaty revolve around several controversial yet basic issues, including the definition of īterrorismī. For example, what distinguishes a "terrorist organisation" from a 'liberation movement'? And do you exclude activities of national armed forces, even if they are perceived to commit acts of terrorism? If not, how much of this constitutes 'state terrorism'?"[37] The coordinator of the negotiations, supported by most western delegations, proposed the following exceptions to address those issues:
"1. Nothing in this Convention shall affect other rights, obligations and responsibilities of States, peoples and individuals under international law, in particular the purposes and principles of the Charter of the United Nations, and international humanitarian law.
2. The activities of armed forces during an armed conflict, as those terms are understood under international humanitarian law, which are governed by that law, are not governed by this Convention.
3. The activities undertaken by the military forces of a State in the exercise of their official duties, inasmuch as they are governed by other rules of international law, are not governed by this Convention.
4. Nothing in this article condones or makes lawful otherwise unlawful acts, nor precludes prosecution under other laws."[38]
I do not disagree that the shooting down of that aircraft was a gross error of judgement.
I strongly disagree that it was a terrorist act. It was not murder. It was an error of judgement with tragic results. Whether you choose to accept it or not the CO of the USS Vincennes believed that his vessel and crew were under threat of attack. He reacted to protect his crew, as he was trained, paid and expected to do.