Just happened to stumble on this (for what it's worth) It comes from an article from Vietnam on the Boston Terror attack:
The attack is being treated as a terrorist act because it involved multiple explosive devices, though investigators have not yet found any link to an organized terror group, foreign or domestic, said a White House official who asked for anonymity. Obama did not use the term terrorism in his televised remarks.
Federal law defines terrorism as violence to “intimidate or coerce” either the government or members of the public, according to the FBI website. The site chosen for the explosions, near the finish line of the race, provided maximum opportunity for television exposure.
Thanks for the info. Very nuanced definition but clears it up a little.