Terrorism is a premeditated, usually politically motivated, use, or threatened use, of violence, in order to induce a state of terror in its immediate victims, usually for the purpose of influencing another, less reachable audience, such as a government.
to induce a state of terror in its immediate victims
So... It's not a terrorist attack until somebody surveys all the immediate victims and asks them whether they felt "intense, sharp, overmastering fear", which is how "Terror" is defined, I believe.
Also the words seem to be making it inclusive of all the immediate victims. So technically, if there is at least one immediate victim who didn't feel the aforementioned fear then this could not be described as a terrorist attack, but rather as an attempt of terrorism.
Allow me to put the silly discussion to rest.
A metre is a unit of measurement, with an absolute objective value defined specifically.
Terror is a subjective state of being, with no scale or set objective reference point. Thus, just like defining anything that pertains to subjective feelings of specific people, the definition will never be specific and discrete. So any definition of "terror" or "terrorism" you will ever come up with will always ultimately be bullshit.