An "act of war" is anything a state decides to go to war over. Given that these assassinations have been going on for several years, evidently not.

Then perhaps there is a third category: an act of terror.

Why do you have to make it into something else? Just call it an assassination.

While the act may induce terror, it's just confusing to lump it in with attacks against civilians. Attacks against military or para-military or chain of command elements are different because people can choose not to join up or to engage in those professions.

Intelligence agencies have been sending "messages" to one another for decades. It's a known hazard of the job.

It could be grounds for either. Whether an act is grounds for war doesn't necessarily mean that the country is in a position to strike back. If Israel did this, Iran probably couldn't afford to strike back militarily without incurring the full wrath of Israel's military. They'd also have to be extremely lucky to not have the US get involved as well.

Does it matter? They have no recourse.

Is Iran's intervention in Syria a crime or an act of war?