Adaptation is one of the central concepts in evolutionary theory, which nonetheless has been given different definitions. Some scholars support a historical definition of adaptation, considering it as a trait that is the outcome of natural selection, whereas others support an ahistorical definition, considering it as a trait that contributes to the survival and reproduction of its possessors. Finally, adaptation has been defined as a process, as well. Consequently, two questions arise: the first is a philosophical one and focuses on what adaptation actually is; the second is a pedagogical one and focuses on what science teachers and educators should teach about it. In this article, the various definitions of adaptation are discussed and their uses in some textbooks are presented. It is suggested that, given elementary students' intuitions about purpose and design in nature and secondary students' teleological explanations for the origin of adaptations, any definition of adaptation as a trait should include some information about its evolutionary history.