Back in 1998, the group Earth Liberation Front (a.k.a. ELF) set a series of fires at Vail ski resort in Colorado and caused $12 million in damage. Authorities at the time called it the most expensive "ecoterrorism" to date.
Burning stuff down is not an activity I personally condone (unless we're talking about Burning Man), but I think the term "arson" is more applicable to ELF's activities in Colorado and elsewhere. The group was protesting Vail's ongoing expansion into Canada Lynx habitat; they didn't kill or hurt anyone. Before people go haywire over misguided monkeywrenching in the West, they should consider what goes on in reverse in Brazil, according to a forthcoming report from Brazil's Catholic Land Commission detailed in the Guardian:
... at least 260 people, among them a Catholic bishop, live under the threat of murder because of their fight against a coalition of loggers, farmers and cattle ranchers.
Save the moniker eco-terrorist for someone who puts a hit on an activist for the work he does to protect rainforests and indigenous people.