In this paper, we examine different forms of altruism. We commence by analysing the behavioural definition and, after clarifying its conditions for altruism, we argue that it is not in reflective equilibrium with everyday linguistic usage of the term. We therefore consider a psychological definition, which we likewise refine, and argue that it better reflects ordinary language use. Both behavioural and psychological approaches define altruism descriptively and thus fail to capture an important aspect of altruism, namely its normative component. Altruism, we argue, is a thick concept, i.e. one which embodies both positive and normative components. We discuss and compare various formulations of this normative component.