The term “speciesism” has played a key role in debates about the moral consideration of nonhuman animals, yet little work has been dedicated to clarifying its meaning. Consequently, the concept remains poorly understood and is often employed in ways that might display a speciesist bias themselves. To address this problem, this article develops a definition of speciesism in terms of discrimination and argues in favor of its advantages over alternative accounts. After discussing the key desiderata for a definition of discrimination (...) in general and speciesism in particular, a wide and evaluative account of speciesism is presented. According to it, speciesism is unjustified consideration or treatment of those who are not classified as belonging to a certain species. The article then discusses rival accounts, namely narrow and descriptive definitions of speciesism. The article argues that such accounts are problematic as they overly restrict the scope of phenomena that can be identified as speciesist and disrupt the continuity with other forms of discrimination such as racism and sexism. (shrink)