Second language (SL) teacher educators tirelessly teach others how to teach. But how often do we actually define teaching? Without explicit definitional activity on this fundamental concept in second language teaching (SLT), it remains implicit and intuitive--the opposite of clear, productive understanding. I therefore explore the question, "What is teaching?" in this paper. First, I establish the claim that the SLT literature rarely defines teaching explicitly, in part because of its technical "how-to" focus. Second, I offer a heuristic definition of teaching as evolutionarily adaptive behavior (TEAB)--as existing in humans because it enables adaptation to varied and complex ecosocial circumstances. In contrast, animals have quite modest adaptive powers, so it may come as a surprise that TEAB is not uniquely human. Therefore, third, I review research comparing animal and human teaching in order to help us understand the latter better. Fourth, I describe teaching as studied by anthropologists--as it varies across human groups. Formal teaching is relatively rare from an anthropological perspective, and relatively recent at that. Fifth, I employ the results of this definitional exercise to exploratorily examine what happens in SLT classrooms. Finally, I discuss implications and future directions for the ideas presented here and conclude.