Any event with multiple explosive devices -- as this appears to be -- is clearly an act of terror ...
Said official was later cited/quoted by Secretary of Defense Chuck Hagel, indicating that this is the presidential "line" on the subject.
I understand why President Barack Obama would want to be "clear" that the Boston Marathon attack was "clearly" a terrorist incident. After all, he got pilloried taking his time before calling the Benghazi attack, which was under no plausible definition a terrorist incident, a terrorist incident.
Since then, he's buckled down assiduously to the post 9/11 American political convention of screaming "IT'S TEH TERRORISTS! RUN FOR YER LIVES!" whenever anyone hears an explosion or sees an Arab in the vicinity.
BUT: It is far from "clear" that the Boston Marathon attack was terrorism.
And no, the criterion of terrorism is not "any event with multiple explosive devices." Although that would be interesting insofar as such a definition would make virtually every US military artillery gunner since the Korean War a terrorist.
So anyway, "terrorism" has two components -- target and objective.
The targets of a terrorist attack are civilian non-combatants. If the targets of an attack are combatants -- military personnel or employees of the government fielding those military personnel -- it's not terrorism.
The objective of a terrorist attack is to create terror, usually but not always for the purpose of affecting government policy. If that's not the objective of the attack, it's not terrorism.
What do we know about the Boston Marathon attack? Not much.
We don't know whether a war-type scenario even applies, so we don't know whether a combatant/non-combatant distinction exists.
This may have been a workaday crime of some sort -- for example, the blasts may have been intended to distract from a robbery to occur elsewhere, or they may have been intended to kill or injure some specific person or persons with whom the bomber(s) had a beef, with the rest of the casualties being "collateral damage." If that's the case, it was not terrorism. That doesn't mean it wasn't an evil crime. It's just that words, you know, mean things.
It is certainly possible that this was a war-like scenario and that all those civilian non-combatants were the targets, and that by killing and injuring those targets the bomber(s) hoped to create terror. In fact, it's not just possible and not just plausible, but seems likely.
But unless Obama et. al know some things they aren't telling us (which is very likely, but that doesn't lend itself very well to the proposition of clarity, does it?), it is far from "clear" that this is the case.
We won't know any of the things we don't know until we know them. The unnamed government official's alleged clarity on the subject is either based on undisclosed information (in which case we can't share his clarity) or else is just a BS propaganda line.