A person who uses the unsubstantiated beliefs and sentiments of environmentalism to impose their political will onto the scientifically illiterate public to inflate their megalomanic sense of power. Usually at the public's detriment.
Eco fascists are the de facto crusaders of what has become the religion of the American political left; that is environmentalism. They use the guise of dogooderism and the pretense of being some kind of environmental savior to justify their economically destructive actions much the same way Hitler did to justify his actions.
Nancy Pelosi is acting as an eco-fascist by voting down congressional efforts to allocate water resources to California farmers who have historically been given access to these resources in favor of the delta smelt (a fish) that had been put on the "threatened" species list; thus favoring the fish over humankinds need to grow food.
Al Gore is one of the most notable and dogmatic eco-fascists for preaching his prophecy of "Truth" for his own self-agrandizement and his obvious agenda of demonizing the oil barons that defeated his political aspirations to the US Presidency, which he clearly felt entitled to.