The term “effective altruism” has no official definition, meaning that different authors will inevitably understand the term in different ways. Since this harbours the potential for considerable confusion, William MacAskill, one of the leaders of the effective altruism movement, has contributed a chapter aimed at forestalling some of these potential confusions. In this chapter, MacAskill first outlines a brief history of the effective altruism movement. He then proposes his preferred definition of “effective altruism”, aiming to capture the central activities and concerns of those most deeply involved in the movement. Finally, he replies to various common misconceptions about the movement. These include the views that effective altruism is just utilitarianism, that it is purely about poverty alleviation, that it is purely about donations, and that it in principle ignores possibilities for systemic change.