What is terrorism?
I’ve been skeptical about the talk of "eco-terrorism" because, to me, a crucial ingredient of anything worthy of the term is deliberate targeting of civilians for injury or death. Since the alleged "eco-terrorists" explicitly aim to avoid any harm to a human being, "terrorist" seems a misnomer.
But am I right about this? Is there a commonly accepted definition of "terrorism"?
I suspect the DOJ has one in mind, given Gonzalez’s very specific language in his press conference: The perps "worked together with extensive planning to influence the conduct of government and private businesses through the use of coordinated force, violence, sabotage, intimidation, and coercion."
The Wikipedia page on the definition of terrorism is instructive:
It also has this:
While the United Nations has not yet accepted a definition of terrorism , the UN’s "academic consensus definition," written by terrorism expert A.P. Schmid and widely used by social scientists, runs:
Terrorism is an anxiety-inspiring method of repeated violent action, employed by (semi-) clandestine individual, group or state actors, for idiosyncratic, criminal or political reasons, whereby — in contrast to assassination — the direct targets of violence are not the main targets. The immediate human victims of violence are generally chosen randomly (targets of opportunity) or selectively (representative or symbolic targets) from a target population, and serve as message generators. Threat- and violence-based communication processes between terrorist (organization), (imperiled) victims, and main targets are used to manipulate the main target (audience(s)), turning it into a target of terror, a target of demands, or a target of attention, depending on whether intimidation, coercion, or propaganda is primarily sought," (Schmid, 1988). 
In November 2004, a UN panel described terrorism as any act: "intended to cause death or serious bodily harm to civilians or non-combatants with the purpose of intimidating a population or compelling a government or an international organization to do or abstain from doing any act." 
So, the UN seems basically to agree that terrorism involves violence against people.
But look at this, from a variety of U.S. government agencies:
U.S. Code of Federal Regulations: "…the unlawful use of force and violence against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives" (28 C.F.R. Section 0.85).
Current U.S. national security strategy: "premeditated, politically motivated violence against innocents."
United States Department of Defense: the "calculated use of unlawful violence to inculcate fear; intended to coerce or intimidate governments or societies in pursuit of goals that are generally political, religious, or ideological."
USA PATRIOT Act: "…acts dangerous to human life that are a violation of the criminal laws of the United States or of any State" (SEC. 802.)
The U.S. National Counter Terrorism Center (NCTC) described a terrorist act as one which was: "premeditated; perpetrated by a subnational or clandestine agent; politically motivated, potentially including religious, philosophical, or culturally symbolic motivations; violent; and perpetrated against a noncombatant target." 
Four of those five include persons subject to violence.
So, not much of a conclusion to draw here. Whether or not you think terrorism must involve deliberate harm to civilians depends on whose definition you accept — or how your personal values line up. For my part, acts of destruction that harm no one do not deserve the term.