1.) A person in a leadership role, usually of limited intelligence and lacking keen self-awareness, who stubbornly insists on making all the decisions that often end in abject failure with dire consequences for others.
2.) George W. Bush's description for himself, at a White House press briefing on April 19, 2006. Bush told reporters, "I’m the decider, and I decide what’s best. And what’s best is for Don Rumsfeld to remain as the secretary of defense.”
I'll jump off this bridge and even take you with me, if I want, because, dammit, I'm the decider. So shaddup, and give me your hand.