The State department's definition of terrorist attacks, quoted in a
NYTimes article of June 22nd, 2004:

"premeditated, politically motivated violence perpetrated against
 noncombatant targets by subnational groups or clandestine agents,
 usually intended to influence an audience."

Kofi Annan's proposed definition of terrorism for the UN, quoted in a
NYTimes article of March 21st, 2005:

  Mr. Annan called for a definition of terrorism as any act "intended
  to cause death or serious bodily harm to civilians or noncombatants"
  aimed at intimidating governments, populations or international
  organizations. "We must convince all those who may be tempted to
  support terrorism that it is neither an acceptable nor an effective
  way to advance their cause," he said.